MANIPULATION 


OF    THE 


MICROSCOPE 


BY 


EDWARD  BAUSCH. 


LUSTRATED. 


TENTH  THOUSAND. 


PUBI.ISHKD    BY 

BAUSCH    &    LOMB    OPTICAL    COMPANY, 
ROCHESTER,    N.  Y. 


Copyright. 

BAUSCH   &   LOMB   OPTICAL  COMPANY. 
1891. 


PREFACE    TO    FIRST    EDITION. 


It  may  seem  to  some  persons  an  act  of  presump- 
tion for  a  maker  of  microscopes  and  microscopic 
accessories  to  enter  the  field  of  authorship  and  attempt 
to  supplement  the  valuable  labors  which  in  recent 
years  have  made  the  use  of  the  microscope  an  indis- 
pensible  aid  in  the  advancement  of  science. 

To  such,  if  any,  I  submit,  that  being  a  producer  of 
microscopes  and  their  accessories,  I  have  had  oppor- 
tunity to  become  acquainted  with  the  lack  of  general 
knowledge  of  the  fund  imental  principles  of  the 
instrument  and  the  best  method  of  technique,  even 
among  owners  of  microscopes  Indeed,  with  so  many 
complications,  with  almost  unlimited  powers  and  uses 
of  the  instrument,  the  beginner  cannot  fail  to  feel  the 
need  of  a  guide  and  adviser. 

In  order  to  accomplish  the  greatest  good,  I  have 
started  out  in  this  little  Manual  with  the  supposition 
that  the  purchaser,  or  owner,  is  a  beginner,  and  abso- 
lutely ignorant  of  the  microscope  and  everything 
which  pertains  to  it,  and  therefore  have  attempted  to 
convey,  step  by  step,  in  as  simple  language  as  I  could 
command,  information  which  will,  I  trust,  lead  to  ease 
of  manipulation  and  give  both  pleasure  and  profit  to 
those  for  whom  it  was  specially  written. 

With  these,  its  purposes  and  hopes,  I  beg  for  my 
self-imposed  labor  a  friendly  reception. 

EDWARD  BAUSCH. 


PREFACE    TO    SECOND    EDITION. 

The  demand  for  this  book  having  considerably 
exceeded  the  expectations  of  its  author,  and  the 
comments  on  its  utility  having  been  so  favorable 
leads  to  the  view  that  it  fills  a  gap  in  microscopi- 
cal literature. 

In  preparing  for  a  new  edition  an  opportunity  has 
been  given  for  enlarging  on  some  of  the  subjects  and 
rewriting  others,  so  as  to  make  them  conform  to  the 
changes  which  the  last  five  years  have  brought  about 
in  the  construction  of  apparatus. 

While  it  may  be  true  that  many  of  the  subjects 
might  be  treated  much  more  extensively,  the  writer 
has  purposely  refrained  from  doing  so,  because  he 
has  considered  it  beyond  the  province  of  his  inten- 
tion and  because  books  giving  more  extensive  infor- 
mation are  available. 

An  intending  purchaser  of  a  microscope  finds  it 
more  or  less  difficult  to  make  a  suitable  selection, 
and  while  it  is  always  best  to  consult  an  exper- 
ienced microscopist,  the  writer  has  endeavored  to 
convey  information  which,  he  hopes,  will  aid  in 
this  direction. 

May,   1891.  THE  AUTHOR. 


CONTENTS. 


SIMPLE   MICROSCOPES. 

Purpose  of  the  Microscope  ;  Kinds  of  Microscopes  ;  Magni- 
fying Power  ;  Using  Magnifiers  ;  Aberrations  ;  Achroma- 
tism, ..---._  PAGES  9-13. 

THE   COMPOUND   MICROSCOPE. 

Description  of  Parts  ;  Base  or  Foot ;  Pillar  ;  Arm  ;  Body  ; 
Nose-Piece ;  Society-Screw  ;  Objectives  ;  Eye-Piece  or 
Ocular  ;  Draw-Tube  ;  Collar  ;  Coarse  Adjustment ;  Milled 
Heads ;  Fine  Adjustment ;  Stage  ;  Clips ;  Centering- 
Screws  ;  Mirror  ;  Mirror-Bar  ;  Sub-Stage  ;  Sub-Stage  Bar  ; 
Diaphragm  ;  Optical  Axis  ;  Object ;  Slide  ;  Cover-Glass  ; 
Classification  of  Microscopes  ;  Mechanical  Parts  ;  Stage  ; 
Mirror  and  Mirror-Bar  ;  Diaphragm,  PAGES  14-25. 

OBJECTIVES   AND   EYE-PIECES. 

Objectives  —  Classes  ;  Systems  ;  Angular  Aperture  ;  Achroma- 
tism ;  Resolving  Power  ;  Flatness  of  Field  ;  Penetration  ; 
Working-Distance;  Magnifying  Power  ;  Eye-Piece — Huy- 
ghenian  ;  Solid  Eye-Piece  ;  Periscopic  Eye-Piece  ;  Nomen- 
clature ;  Flatness  of  Field  ;  Size  of  Field,  PAGES  26-46. 

REQUISITES    FOR   WORK. 

Working  Table  ;  Room  ;  Light;  Position  of  Light  ;  Which  Eye 
to  Use  ;  The  Ward  Eye  Shade  ;  Order  ;  Material, 

-     PAGES  47-^51. 

HOW   TO  WORK. 

To  Set  Up  the  Instrument ;  Centering  Stage  ;  Illumination  ; 
Attaching  High  Power  Objectives  ;  How  to  Work  ;  Dark 
Ground  Illumination;  Cover-Glass;  To  Draw;  Camera 
Lucida — How  to  Use  It;  Determining  the  Magnifying 
Power, -  PAGES  52-73. 


ADVANCED    MANIPULATION. 

Dry-Adjustable  Objectives  ;  Immersion-Adjustable  Objectives  ; 
Test-Plate;  Immersion  Objectives  on  Test-Plate;  Photo- 
Micrography,  ------  PAGES  74-89. 

TO  SELECT    A    MICROSCOPE. 
Stands  ;  Objectives  and  Eye-Pieces  ;  Accessories, 

PAGES  90-101. 

SUB-STAGE    ILLUMINATION. 

Mirror  ;  Abbe  Condenser  ;  Centering  ;  To  Focus  Condenser  ; 
Intensity  of  Light,  PAGES  102-106. 

CARE   OF   A    MICROSCOPE. 

To  Take  Care  of  a  Stand  ;  To  Take  Cafe  of  Objectives  and 
Eye-Pieces,  PAGES  107-110. 

APPENDIX. 
Considerations  in  Testing  Objectives,         -     PAGES  111-118. 


SIMPLE  MICROSCOPES. 


Purpose  of  the  Microscope.— The  Microscope 
is  an  instrument  which  magnifies  objects,  so  that  we 
are  better  able  to  examine  their  structure  than  is  pos- 
sible with  unassisted  vision. 

Kinds  of  Microscopes. — Microscopes  may  be 
divided  into  two  classes — simple  and  compound — the 
difference  between  the  two  being  that  with  the  former 
the  object  is  viewed  directly,  while  with  the  latter  a 
magnified  image  is  observed  ;  while  the  first  shows 
the  objects  in  their  true  position,  the  latter  shows 
them  reversed,  so  that  what  is  right  in  the  object  is 
left  in  the  image,  and  when  an  object  appears  to  be 
moving  in  a  certain  direction,  the  movement  is  in 
reality  the  reverse,  and  must  be  moved  accordingly 
to  keep  it  in  view. 


Fig.  i. 

Magnifiers.  —  Simple  microscopes  are  usually 
termed  magnifiers,  and,  when  consisting  of  one  or 
more  lenses,  always  remain  simple.  The  most  com- 


10 

mon  are  those  with  one  or  several  double-convex 
lenses  (Fig.l).  The  shorter  the  radii  (the  more  curved 
the  surfaces)  are  in  these,  the  greater  will  be  the  mag- 
nifying power,  and  the  higher  this  is,  the  less  of  the 
object's  surface  can  be  seen  at  once.  Each  addi- 
tional lens  increases  the  magnifying  power  in  pro- 
portion to  its  curvature.  The  distance  between  the 
lens  and  the  object,  when  this  is  seen  most  distinctly, 
is  called  the  focus  ;  at  the  point  where  the  object  is 
most  distinct,  the  lens  is  said  to  be  in  focus  ;  when 
indistinct  or  blurred,  out  of  focus. 

Magnifying  Power. — Unless  a  microscope  is 
known  to  come  from  the  hands  of  a  reliable  firm,  any 
claim  as  to  magnifying  power  should  be  accepted  with 
reserve.  In  former  years,  when  the  country  was  over- 
run with  cheap  foreign  productions,  the  most  fanciful 
claims  were  made  in  this  direction.  It  is  evident  that 
a  lens  magnifies  an  object  equally  in  all  directions  ; 
this  is  said  to  be  in  areas,  and  is  the  square  of  the 
linear,  so  that  if  an  object  is  magnified  4  times  in 
the  linear,  it  is  16  times  in  area.  The  commonly 
accepted  term  to  express  magnifying  power  of  sim- 
ple, as  well  as  compound  microscopes,  is  in  diameters 
(linear}.  A  single  lens  of  1  inch  focus  magnifies 
about  ten  diameters  ;  one  of  2  inch  focus,  about  5 
diameters  ;  one  of  \  inch  focus,  20  diameters,  and  so 
on.  In  a  lens  of  high  magnifying  power,  the  focus  is 
ordinarily  about  twice  the  diameter,  so  that  if  a  lens 
is  \  inch  diameter  its  focus  is  about  1  inch.  This 
may,  however,  be  more  accurately  determined  by  pro- 


11 

jecting,  say  a  flame  or  window  frame,  upon  a  white 
piece  of  paper ;  the  distance  between  the  paper  to  the 
center  of  the  lens,  when  the  image  is  most  distinct,  is 
its  focal  distance.  When  a  lens  is  two  inches  or  more 
in  diameter,  it  is  usually  .termed  a  reading  glass. 

Using  Magnifiers. — In  using  magnifiers  the  lens 
should  be  held  close  to  the  eye  and  such  a  position 
taken  that  the  object  will  receive  the  best  illumina- 
tion. In  the  lenses  of  equally  convex  surfaces,  it  is 
immaterial  which  side  is  held  toward  the  eye  ;  but 
when  plano-convex  lenses  are  used,  the  plane  side 
should  always  be  toward  the  eye,  as  it  gives  the  flat- 
test field. 

Aberrations. — Two  factors  arise  which  prevent 
the  advantageous  use  of  more  than  about  25  diame- 
ters in  magnifiers ;  they  are  called  the  chromatic  and 
spherical  aberrations.  The  first  is  the  term  employed 
when  the  object  is  apparently  fringed  with  color,  pre- 
dominently  blue  and  yellow  ;  the  second,  when  all  but 
the  central  portion  of  the  lens  shows  the  object  indis- 
tinctly ;  these 
faults  increase 
with  the  magni- 
fying power.  In 
the  case  of  a 
combination  of 
Fig-  2-  several  lenses,  Fis-  3- 

they  may  partially  be  overcome  by  interposing  an 
opaque  plate  with  a  small  opening,  called  a  diaphragm, 
between  them,  which  cuts  off  the  outer  or  marginal 


12 

rays,  or  the  lenses  may  be  made  of  a  smaller  diam- 
eter. An  incision  may  also  be  cut  into  the  glass 
equally  between  the  two  surfaces,  when  from  the  name 
of  the  inventor,  it  is  called  a  Coddington.  Fig.  2  shows 
a  section  of  a  Coddington,  while  Fig.  3  shows  it  in 
its  mounting. 

Achromatism. — The  most  approved  method, 
however,  for  eliminating  these  appearances,  is  by  the 
use  of  one  or  two  con  cave  flint  glass  lenses  in  connec- 
tion with  the  double  convex  crown  glass  lens.  When 
the  color  or  chromatic  aberration  is  thus  removed,  the 
lenses  are  said  to  be  achromatic,  and  when  both  the 
chromatic  and  spherical  aberrations  are  avoided,  the 
lens  is  called  aplanatic,  and  is  then  said  to  be  cor- 
rected. An  achromatic  lens,  composed  of  one  flint 
and  one  crown  glass  lens,  is  called  a 
doublet  (  Fig.  4 )  ;  one  with  two  flint 
glass  lenses  and  one  crown  glass  is 
Fig.  4.  called  a  triplet  (Fig.  5).  The  latter  is 
the  best  form,  as  it  gives  the  highest  cor- 
rection ;  such  a  lens  (it  is  thus  called  from 
the  the  fact  that  the  lenses  are  cemented 
together  and  act  like  one)  may  be  held 
with  either  side  toward  the  object  with 
equally  good  results,  and  may  also  be  held  Fig.  5. 
at  quite  an  obliquity,  without  loss  of  definition  ;  this 
feature  is  important,  as  it  is  almost  impossible  to  give 
a  lens  a  theoretically  correct  position  to  both  the  eye 
and  object  with  the  unaided  hand. 


Fig.  6. 


THE  COMPOUND  MICROSCOPE. 


As  was  previously  stated  a  magnified  image  is 
observed  in  the  Compound  Microscope.  Any  two 
lenses,  one  of  short,  the  other  of  long  focus,  placed 
sufficiently  far  apart,  will  attain  this  object,  and  this 
was  for  years  the  method  of  its  construction. 

On  any  microscope,  whether  simple  or  compound, 
the  difficulty  of  holding  it  or  the  object  steady  during 
observation  increases  with  the  increase  in  magnifying 
power,  and  in  the  compound  form  with  only  a  mod- 
erately high  power,  it  is  utterly  impossible  to  retain 
sufficient  steadiness  to  make  any  reliable  observation. 
Mechanical  contrivances  thesefore  became  a  neces- 
sity, and  were  applied  in  the  earliest  constructions  of 
the  microscope.  They  were  all  made  to  fulfill  the  fol- 
lowing three  conditions :  A  platform  for  holding  the 
object ;  a  means  of  adjustment  for  properly  focusing 
on  the  object,  and  a  mode  of  suitably  illuminating  the 
same. 

From  what  may  now  be  called  a  crude  attainment  of 
these  three  purposes,  the  construction  gradually 
became  more  complex.  Many  additions  have  been 
made  which  have  proven  useful  and  have  remained, 
while  others  have  been  discarded,  and  these  have  led 
to  the  present  construction  of  the  microscope. 


15 

While  certain  parts  are  necessary  to  make  up  a 
modern  instrument,  no  one  design  of  construction  is 
followed.  The  forms  are  innumerable,  each  maker 
following  his  own  inclination  in  variety,  design,  num- 
ber of  parts  and  material.  For  the  latter  brass  pre- 
dominates, although  bronze  and  iron  are  used  to  a 
considerable  extent.  The  first  two  metals  are  usually 
highly  finished,  and  as  they  easily  tarnish  in  this 
state,  are  protected  by  lacquer,  which  is  not  only  ser- 
viceable in  this  direction  when  well  done,  but  offers 
a  means  of  ornamentation.  The  latter  metal  is  cov- 
ered with  a  heavy  coating  of  japan  and  being  an 
intense  black  is  on  this  account  often  recommended 
by  instructors  as  being  agreeable  for  the  eyes.  The 
entire  apparatus  is  called  a  microscope,  whereas,  with- 
out the  optical  parts,  it  is  termed  a  stand. 

Description  of  Parts. — As  it  is  necessary  for 
the  student  to  become  conversant  with  the  terms  of 
the  various  parts  and  to  understand  their  use,  we  give 
an  illustration  (Fig.  6)  with  letters,  and  append  a  list 
giving  the  names. 

A.  Base  on  Foot. — This  is  the  foundation  of 
the  instrument.  It  usually  rests  upon  three  points  (or 
should  do  so)  and  is  of  such  a  weight  that  it  keeps 
the  instrument  firm  when  it  is  in  an  upright  or  inclined 
position.  The  revolving  plate,  when  this  is  provided, 
by  means  of  which  the  upper  portion  of  the  instru- 
ment is  revolved,  without  changing  the  position  of 
the  base,  is  considered  a  part  of  it. 


16    . 

B.  Pillar. — It  is  that  portion  which  is  fastened  to 
the  base  and  may  be  one  or  two,  according  to  the 
construction  of  the  stand.     It  carries  upon  its  upper 
end  the  joint  or  axis. 

C.  Arm. — This  is  connected  with  the  pillar  by  the 
joint  and  supports  all  theworhing  parts  of  the  instru- 
ment. 

D.  Body. — This  is  the  tube-portion  to  which  the 
optical  parts  are  attached. 

E.  Nose-Piece. — This  is  an  extra  piece  which  is 
attached  to  the  lower  part  of  the  tube. 

Society  Screw. — This  is  a  standard  screw  which 
is  cut  into  the  nose-piece,  and  is  called  so  from  the 
fact  that  it  was  first  established  by  the  Royal  Micro- 
scopical Society  of  London.  It  is  also  called  the  /////- 
versa!  screw,  and  is  in  general  use  in  this  country  and 
England  ;  it  has  lately  been  adopted  by  some  firms  on 
the  Continent  of  Europe. 

F.  Objectives. — This  is  screwed  into  the  nose- 
piece  and  is  called  so  because  it  is  nearest  the  object. 
It  is  the  most  important  of  the  two  optical  parts  (of 
the  microscope  proper)  and  upon  its  perfection  the 
distinctness  of  the  image  and  therefore  the  value  of 
the  instrument  almost  entirely  depends. 

G.  Eye-Piece  or  Ocular. — It  is  called  so  because 
it  is  nearest  the  eye  and  is  the  remaining  optical  part. 
It  magnifies  the  image  given  by  the  objective.     This 
and  objective  will  be  treated  more  fully  later  on. 


17 

H.  Draw-tube. — This  is  that  portion  of  the  body 
which  moves  in  the  outer  sheath  and  which  receives 
the  eye-piece.  It  is  provided  for  the  purpose  of  attain- 
ing different  lengths,  variations  in  magnifying  power 
and  as  a  matter  of  convenience  while  working. 

I.  Collar. — This  is  a  ring  which  is  attached  to  the 
draw-tube  and  is  usually  provided  with  a  milled  edge. 

J.  Coarse   Adjustment.— This   is   a   provision 

for  moving  the  body  quickly  back  and  forth  for 
adjusting  the  focus  approximately.  It  is  done  by 
a  sliding  rack  and  stationary  pinion  (not  shown  in 
cut )  or  a  sliding  body  in  an  outer  sheath. 

K.  Milled  Heads. — These  are  attached  to  the 
shank  of  the  pinion,  which  is  revolved  by  means  of 
them  and  are  usually  large  to  give  sensitiveness  to 
the  movement. 

L.  Fine  Adjustment. — This  is  slow  moving  and 
serves  to  get  an  exact  focus.  It  is  attained  by  a  fine 
thread,  provided  with  a  milled  head,  and  acts  upon 
the  body,  either  directly  or  by  levers.  This  as  well 
as  the  coarse  adjustment  should  be  extremely  sensi- 
tive and  should  not  have  the  least  side  or  lateral 
motion.  The  fact  that  either  of  them  have  it,  is 
evidence  of  poor  workmanship. 

M.  Stage. — This  is  the  portion  on  which  the 
object  is  placed  for  examination  and  is  attached  to 
the  arm. 

N.  Clips. — These  are  two  springs  which  are 
attached  to  the  upper  surface  of  the  stage  and 
serve  to  hold  down  the  object. 


18 

Centering  Screws. — These  are  provided  for 
moving  the  stage  in  different  directions  to  bring 
the  center  of  its  revolving  motion  in  the  center  of 
the  field. 

O.  Mirror. — This  is  used  for  reflecting  and 
condensing  light  upon  the  object.  As  a  rule  two 
mirrors  are  used,  one  plane  and  the  other  concave. 
The  first  gives  a  comparatively  weak  light,  while  the 
second  concentrates  it  and  gives  it  more  intensity. 

P.  Mirror-bar. — This  carries  the  mirror  and  by 
a  sliding  arrangement  allows  the  variations  in  dis- 
tance of  the  mirror  to  the  stage_;  it  also  swings  in 
a  circle  around  the  object  in  order  to  illuminate  it 
from  any  direction. 

Q.  Sub-Stage. — This  is  a  ring  below  the  stage 
to  receive  various  accessories  which  may  be  required. 
It  is  sometimes  fixed  to  the  stage  but  in  the  best 
instruments  it  is  separated  from  it  and  is  provided 
with  an  adjustment  to  vary  its  distance  from  the 
object. 

R.  Sub-Stage  bar. — This  receives  the  sub-stage 
and  permits  its  adjustment.  In  modern  American 
instruments  this,  as  well  as  the  mirror-bar,  is  on  an 
axis  in  the  plane  of  the  stage,  so  that  whatever  posi- 
tion they  may  be  in,  relative  to  the  object,  the  dis- 
tance from  this  to  the  sub-stage  or  mirror  does  not 
vary,  except  when  made  to  do  so. 

S.  Diaphragm. — This  is  a  perforated,  revolving 
disk,  attached  either  to  the  stage  or  sub-stage.  It 


19 

has   holes   of  different  sizes  so  that 
light  from  the  mirror  may  be  modified. 

Optical  Axis. — This  is  an  imaginary  line  which 
passes  from  the  center  of  the  eye-piece  through  the 
body,  objective,  stage  and  sub-stage  to  the  mirror. 
Whatever  lies  in  it  is  said  to  be  centered. 

Object. — That  which  is  examined  and  placed 
upon  a  slide. 

Slide. — This  is  a  thin  plate  of  glass,  generally 
3  inches  long  by  1  inch  wide. 

Cover  Glass. — This  is  an  extremely  thin  piece 
of  glass,  round  or  square,  which  is  placed  upon  the 
object,  either  for  flattening  or  preserving  it,  or  both. 

Classification  of  Microscopes. — Up  to  within 
recent  years  microscopes  have  been  divided  into  two 
classes  :  the  Jackson  and  Ross  models.  While  the 
latter  was  for  many  years  very  popular,  particularly 
with  the  English  makers,  it  has  been  almost  entirely 
superseded  by  the  Jackson  form,  and  with  good 
reason.  In  the  former  the  means  of  adjusting  were 
provided,  as  near  as  consistent  with  the  construction, 
to  the  body  or  tubes,  whereas  in  the  Ross  they  are 
placed  at  the  back  or  more  distant  point  in  the 
instrument,  thus  increasing  by  means  of  the  con- 
necting arm  the  faults  which  might  exist  in  the 
adjustment. 

A  certain  form  of  instrument  which  at  the  present 
is  very  popular  and  called  the  Continental  pattern, 
from  the  fact  that  it  was  made  originally  by  the 


20 

manufacturers  on  the  continent  of  Europe,  is  a  com- 
bination of  both  the  Jackson  and  Ross  models. 
Whereas,  the  coarse  adjustment  when  consisting  of 
a  rack  and  pinion  is  placed  closely  to  the  tubes,  the 
fine  adjustment  is  placed  on  the  arm  and  although 
being  dissimilar  from  the  original  Ross  in  being 
higher,  it  nevertheless  has  the  disadvantage  of  mag- 
nifying any  lost  motion  in  the  adjustment  by  means 
of  the  connecting  arm.  Considering  the  fact  that 
the  Ross  form  alone  is  almost  obsolete  and  many 
instruments  of  the  present  day  are  a  combination  of 
both  forms,  it  appears  to  the  writer  that  their  desig- 
nations have  lost  their  value. 

There  is  another  direction,  however,  in  which 
microscopes  are  divided  into  two  classes,  which  is 
of  far  more  import ance,  and  affects  their  utility  in 
a  much  higher  degree.  The  writer  does  not  know 
that  instruments  have  been  so  classified  by  others, 
and  knows  that  the  subject  has  been  given  no 
important  significance. 

In  the  Continental  form  just  mentioned,  a  short 
tube  from  160.0  to  170.0  mm.  (0.3  to  6.7  inch)  is 
used,  whereas  in  the  English  form,  and  this  is  largely 
followed  in  America,  the  length  is  from  8|  to  10  in. 
(216.0  to  250.0  mm.).  The  short  tube  of  the  Euro- 
pean makers  offers  no  optical  advantages,  but  is 
mainly  used  to  contract  the  height  of  the  instrument 
to  as  great  an  extent  as  possible,  as  this  is  the  vital 
point  throughout  its  construction. 


21 

At  the  last  meeting  of  the  American  Society  of 
Microscopists  a  committee  was  appointed  to  consider 
the  tube  lengths  as  well  as  other  subjects  to  be  men- 
tioned hereafter  and  reported  in  favor  of  the  adoption 
of  two  standards  for  tube  lengths,  160.0  mm.,  or  6.3 
inch  for  the  short  one,  and  8|  inch,  or  216.0  mm., 
for  the  long  one.  The  American  makers  have 
adopted  these  two  lengths  and  we  believe  are  gen- 
erally following  them.  Practically,  there  are  no 
advantages  in  one  or  the  other,  except,  perhaps,  in 
so  far  as  the  short  tube  might  be  considered  advan- 
tageous, but  optically  this  recommendation  of  the 
committee  is  far  reaching,  because  an  objective,  par- 
ticularly in  giving  considerable  magnification,  when 
constructed  to  be  used  with  a  certain  tube-length, 
should  be  used  with  it  only.  When  used  with  the 
other  standard  it  will  fail  to  give  satisfactory  results. 
This  subject,  with  the  optical  results,  will  be  touched 
upon  again  later  on. 

Mechanical  Parts — As  there  will  be  but  little 
occasion  to  recur  to  the  mechanical  portions  of  the 
microscope,  we  take  the  occasion  to  speak  of  them 
in  a  more  extended  way  in  this  connection. 

Stage. — This  may  be  divided  into  three  classes, 
ordinary,  semi-mechanical,  and  mechanical. 

The  first  consists  of  a  plain  plate,  either  round  or 
square,  on  which  the  slide  is  held  by  means  of  the 
clips.  As  the  microscope  is  used  principally  in  an 
inclined  position,  the  pressure  exerted  by  the  clips 


22 

must  be  sufficient  to  retain  the  slide  firmly  in  posi- 
tion. This  causes  a  certain  amount  of  friction 
between  the  lower  surface  of  the  slide  and  the  upper 
one  of  the  stage,  which  at  times  is  very  annoying,  in 
that  it  is  difficult  to  move  the  object  to  a  point 
desired,  with  certainty.  The  motion  is  "jerky"  and 
disagreeably  harsh  unless  the  stage  is  very  clean. 

A  large  number  of  plans  have  been  devised  to 
overcome  this  difficulty,  which  have  proven  very 
efficient.  They  may  be  termed  semi-mechanical,  in 
that  the  fingers  do  not  come  in  direct  contact  with 
the  slide.  Usually  glass  is  brought  in  contact  with 
the  metal,  but  so  arranged  as  to  offer  as  few  and 
small  points  of  contact  as  possible. 


Fig.  7. 

In  Fig.  7  we  show  a  so-called  glass-stage  and  slide- 
carrier,  in  which  the  stage  is  merely  for  the  support 
of  the  slide-carrier.  This  latter  is  arranged  to 
receive  the  slide  and  rests  upon  the  glass  by  small 
points.  At  its  ends  are  two  projecting  tongues 
which  are  bent  downward  and  inward  and  act  as 
springs  against  the  lower  surface  of  the  glass  plate. 
While  the  movements  must  be  carried  out  by  the 
hands,  they  are  smooth  and  steady  and  work  with 
ease,  even  under  high  powers. 


23 

The  mechanical  stage  is  in  every  sense  mechanical. 
The  hands  do  not  come  in  contact  with  the  slide 
except  to  place  it  on  the  stage.  Two  movements 
are  usually  provided  at  right  angles  to  one  another, 
either  both  with  rack  and  pinion  adjustment,  or  one 
with  rack  and  pinion  while  the  other  has  a  screw 
motion.  Examinations  can  be  carried  out  with  ease 
and  reliability  under  the  highest  powers.  The  main 
value  of  the  mechanical  stage  is  that  systematic 
examinations  over  the  entire  surface  of  an  object 
can  be  carried  on. 

Mirror  and  Mirror-bar. — The  proper  illumi- 
nation of  an  object  is  an  important  feature,  and 
although  there  are  numerous  accessories  for  properly 
accomplishing  this,  which  will  be  spoken  of  later  on, 
the  mirrors  alone  are  effective  agents  when  properly 
constructed  and  applied,  particularly  when  no  high 
magnification  is  used.  The  plane  mirror  is  usually 
used  on  very  low  powers,  and  reflects  light  in  about 
the  same  intensity  as  its  source.  The  concave  mirror, 
however,  is  intended  to  concentrate  the  light  so  that 
all  the  rays  which  strike  its  surface  are  reflected 
toward  its  optical  axis  and  come  together  at  some 
point  above  it,  and  the  rays  from  the  surface  being 
contained  within  a  comparatively  small  space,  cause 
an  increased  intensity,  This  point  is  called  the  focal 
point,  and  is  usually  arranged  to  coincide  with  the 
opening  of  the  stage  when  parallel  rays,  such  as 
from  the  sky,  are  used.  When  the  source  of  light 
comes  considerably  nearer  to  the  mirror,  as  for 


24 

instance  from  a  lamp  and  the  rays  are  diverging,  the 
focal  distance  becomes  considerably  longer,  and  when 
very  close  may  be  twice  as  long.  Some  of  the  intensity 
is  lost  in  consequence,  as  well  as  the  degree  of  con- 
vergence. For  this  reason  mirror-bars  are  so  arranged 
that  the  distance  of  the  mirror  from  the  stage  may  be 
varied  to  accommodate  the  variation  in  the  source  of 
light.  While  this  is  of  considerable  aid,  there  is  not 
sufficient  room  for  a  complete  accommodation,  with 
the  result  that,  under  certain  conditions,  the  utmost 
effectiveness  of  the  microscope  is  not  obtained. 

Diaphragm. — This  is  provided  for  regulating  the 
amount  of  light.  While  the  mirror  should  work  to 
its  utmost  capacity,  it  very  often  occurs  that  for  cer- 
tain investigations  a  profuseness  of  light  is  more 
harmful  than  otherwise.  When  too  much  light  exists, 
objects  are  said  to  be  drowned  in  it,  and  often  makes 
it  impossible  to  determine  structures.  An  intelligent 
use  of  the  diaphragm  is  of  great  service. 

Besides  the  revolving  diaphragm  there  are  a  num- 
ber of  other  forms  which  may  be  said  to  be  better — 
for  instance,  the  so-called  cup  diaphragms,  which 
require  a  separate  piece  for  each  aperture  and  which 
are  held  by  a  special  sub-stage  receiver.  Then  the 
dome  diaphragm,  which  is  a  new  application  of  the 
ordinary  revolving  diaphragm.  It  consists  of  a  sub- 
stage  fitting  having  a  dome  to  which  is  fitted  a 
curved  revolving  diaphragm 

The  ideal  regulator  of  light  is  the  Iris  diaphragm 
consisting  of  a  series  of  overlapping  blades  placed 


25 

around  a  central  opening,  the  size  of  which  may  be 
varied  by  means  of  a  lever  or  milled  edge.  In  the 
ordinary  revolving  form  the  aperture  is  of  necessity 
at  some  distance  from  the  object,  and  does  not  fully 
control  the  light  on  account  of  the  stray  rays,  which 
the  other  three  forms  accomplish. 


OBJECTIVES  AND  EYE-PIECES. 


Although  considerable  magnifying  power  may  be 
attained  by  the  use  of  two  single  lenses  arranged  in 
a  compound  form,  there  is  no  advantage  in  it,  from 
the  fact  that  the  faults  in  the  lenses  are  correspond- 
ingly magnified,  and  these  are  so  considerable  that 
they  destroy  what  it  is  the  purpose  of  the  microscope 
to  give — a  distinct  image. 

Objectives,  Classes. — Objectives  may  be  divided 
into  two  classes,  dry  and  immersion  ;  in  the  former  no 
intervening  medium  except  air  exists  between  the 
cover  and  objective,  while  in  the  latter  a  fluid  is  used 
to  connect  the  upper  surface  of  the  cover  to  the  front 
surface  of  the  objective.  The  use  of  immersion  fluid 
has  several  advantages,  the  first  of  which  is  that  the 
objective  may  be  made  to  give  better  performance,  as 
will  be  explained  later  on  ;  the  second  is  that  more 
light  will  be  transmitted,  as  there  is  less  loss  of  it  by 
refraction. 

It  should  be  understood  however  that  no  advantage 
will  be  gained  by  using  immersion  fluid  with  a  dry 
objective.  It  does  not  increase  its  effectiveness  one 
particle,  on  the  contrary  it  detracts  from  its  quality. 
When  it  is  stated  that  an  immersion  objective  has  a 


2V 

greater  capacity,  it  is  with  the  understanding  that  it 
is  so  constructed  as  to  give  this  result. 

While  many  immersion  objectives  are  constructed 
to  work  both  as  dry  and  immersion,  such  a  plan  can- 
not be  said  to  be  advantageous.  Such  objectives  may 
be  made  to  work  well  in  one  direction  and  be  of 
indifferent  quality  in  the  other,  or  may  be  of  medium 
grade  both  ways.  There  is  no  question  that  the  best 
plan  .is  to  have  each  objective  selected  with  a  view  to 
a  specific  purpose  and  use  it  for  this  purpose  only. 
•  There  are  two  fluids  in  general  use  at  the  present 
time,  water  and  homogeneous  fluid.  The  latter  expres- 
sion means  of  the  same  kind,  and  refers  to  the  fact 
that  the  fluid  has  about  the  same  refractive  and  dispers- 
ive power  as  glass,  so  that  when  this  fluid  fills  up  the 
space  between  the  two  surfaces  of  glass,  a  ray  of  light 
passes  through  the  three  mediums  as  if  they  were  one 
body. 

The  two  large  classes  of  dry  and  immersion  objec- 
tives may  again  be  subdivided  into  two  classes — objec- 
tives for  long  and  short  standard  tube.  As  followed 
by  some  firms  at  present  and  what  it  is  hoped  will 
become  a  universal  custom  in  time,  each  objective  is 
marked  for  the  tube-length  for  which  it  is  corrected 
and  with  which  it  is  assumed  it  will  accomplish  the 
best  results. 

Objectives  are  sometimes  called/<?7t'^;-.r,  and  in  this 
sense  are  divided  into  three  classes :  low,  medium  and 
high.  Dr.  Carpenter  classifies  them  as  follows  :  low 
powers,  3  inch,  2  inch  \\  inch,  1  inch,  f  or  J  inch  ; 


28 

medium  powers,  T4^  inch,  \  inch,  J-  inch,  ^  inch ; 
high  powers,  \  inch,  \  inch,  -^  inch,  r1^  inch,  ^  inch, 
210  inch,  ^g-  inch. 

As  the  objective  is  the  most  important  of  the  two 
optical  parts,  it  follows  that  this  must  be  as  free  from 
faults  as  possible  and  all  that  human  inge- 
nuity and  skill  can  devise  is  utilized  to  attain  this 
end.  The  advance  in  the  perfection  of  the  objective 
has  been  step  by  step  and  each  era  was  at  the  time 
considered  by  many  authorities  the  limit  to  further 
improvement.  Each  advance  was  signalized  by  a 
marked  opposition  and  disbelief  of  its  possibility.  It 
is  therefore  of  inestimable  credit-  to  the  pioneer 
objective-makers,  and  notably  among  these  two  Ameri- 
cans, who  by  quiet  but  stubborn  application  disproved 
previous  claims  and  opened  the  way  to  further  improve- 
ments. A  theoretical  limit  has  been  fixed  on  the 
capacity  of  the  microscope,  which  according  to  our 
present  knowledge  can  not  even  be  reached. 

While  the  introduction  of  water  immersion  made  it 
possible  to  obtain  higher  optical  results  than  with  the 
dry  objectives,  as  will  be  explained  later  on,  the  homo- 
geneous immersion  offers  still  greater  possibilities  in 
this  direction,  and  the  advantages  are  so  pronounced 
that  the  former  are  gradually  coming  into  disuse, 
although  for  certain  kinds  of  work  they  will  be  pre- 
ferred and  used  by  many  persons.  At  present  homo- 
geneous fluid  is  made  of  either  thickened  glycerine 
or  cedar  oil,  and  great  care  is  required  in  keeping 


29 

the  front  of  objectives  and  cover  glasses  properly 
cleaned,  in  which  respect  water  has  the  decided 
advantage. 

It  might  be  stated  that  such  high  power  as  ^th 
and  2Vtn  are  vei7  rarely  constructed  at  the  present  day, 
and  the^th  may  be  considered  the  maximum,  while 
the  TVth  is  that  most  ordinarily  used.  This  power  will 
give  all  the  optical  advantages,  while  higher  powers 
involve  so  many  mechanical  difficulties  as  to  increase 
the  cost  of  production  very  considerably,  and  as  a 
rule  detract  from  the  optical  qualities. 

A  modern  objective  of  the  highest  capacity  may  be 
considered  a  work  of  art,  and  there  are  a  few  pro- 
ductions of  the  human  hand  which  exact  so  much 
untiring  application,  ingenuity  and  skill. 

Systems. — An  objective  is  said  to  consist  of  sys- 
tems which  may  vary  in  number  from  one  to  four 
and  five  ;  two  and  three  are  however  mainly  in  use. 
They  are  the  individual  portions  consisting  of  one, 
two  or  three  lenses,  which  when  more  than  one,  are 
cemented  together  and  make  up 
the  objective.  An  achromatic  sin- 
P  gle  system  may  consist  of  two  or 
three  lenses,  and  a  three  or  four 
system  objective  may  consist  of 
as  many  as  seven  or  eight  lenses. 
The  systems  are  called  in  their 
A  order  :  anterior  or  front,  middle 
and  posterior.  When  one  consists 
of  two  lenses  it  is  called  a  doublet, 
when  of  three  lenses  a  triplet.  Thus  in  Fig.  8?  A  is 


the  anterior,  M  the  middle  and  P  the  posterior 
systems  ;  thus  also  A  is  a  single  system,  M  a  double 
and  P  a  triple  one. 

The  various  features  which  must  be  considered  as 
determining  the  quality  of  an  objective  are  :  angular 
aperture,  achromatism,  resolving  power,  flatness  of 
field,  penetration,  working  distance  and  magnifying 
power.  Although  these  attributes  may  be  considered 
separately,  some  of  them  go  hand  in  hand.  The 
presence  or  extent  of  one  necessarily  involves  or  pre- 
cludes another. 

Angular  Aperture. — The  angle  which  the  most 
extreme  rays,  which  are  transmitted  through  the 
objective,  make  at  the  point  of  focus,  is  called  its 
angular  aperture,  or  in  short  its  angle,  and  of  all  the 
qualities  in  an  ideal  objective, 
this  is  the  most  important.  Thus 
p  in  Fig.  9,  I)  is  considered  the 
point  of  focus,  and  C  D  E  the 
angular  aperture.  The  above 
definition  has  its  limitations, 
however.  While  in  objectives 
of  proper  construction  it  holds 
true,  there  are  many  in  which  it 
is  not  the  case.  For  instance, 
an  objective  may  be  so  con- 
structed that  it  may  transmit  a 
considerable  number  of  rays  in 


81 

excess  of  those  which  combine  to  form  an  image,  and 
it  is  evident  that  these  should  not  be  considered  as 
belonging  to  them. 

As  there  are  many  objectives  of  the  same  power, 
but  of  different  angular  aperture,  there  are  again  oth- 
ers of  varying  power,  but  of  the  same  angle.  Other 
things  being  equal,  it  is  the  angular  aperture  of  an 
objective  which  determines  the  quality.  It  is  expressed 
in  degrees,  and  is  also  spoken  of  as  being  wide,  med- 
ium or  narrow,  although  this  is  indefinite  and 
depends  considerably  upon  the  power  of  the  objec- 
tive ;  while  the  angle  may  be  excessively  wide  for  a 
low  power,  it  may  be  narrow  for  a  higher  one. 

For  many  years  the  extent  to  which  angular  aper- 
ture could  be  carried  was  a  matter  of  controversy,  as 
was  also  the  use  of  objectives  of  wide  and  narrow 
angles  for  different  directions  of  work.  It  is,  how- 
ever, a  matter  of  congratulation  that  the  question  is 
at  rest,  although  it  has  served  a  good  purpose  in  pro- 
mulgating a  better  knowledge  of  the  subject. 

All  objects  emit  rays,  and  it  is  evident  that  those 
coming  from  one  point  and  contained  in  a  large  angle 
are  more  numerous  than  those  in  a  small  angle  ;  also 
that  as  the  angle  more  nearly  approaches  180  degrees 
the  rays  will  be  larger  in  number. 

It  is  assumed  that  two  objects,  B  and  B',  are 
equally  bright,  and  therefore  emit  the  same  number 


of  rays  ;  for  the  purpose  in  hand,  it  is  sufficient  to 
consider  only  those  which  reach  the  plane  surface  of 
the  large  lens  ;  if  an  equal  space  is  imagined  over  B', 
the  same  number  of  rays  will  be  contained  in  this  ; 


C  A 


Fig.  10. 

therefore,  the  cone  contained  in  the  angle  A'  B'  C' 
will  contain  as  many  rays  as  that  contained  in  A  B  C; 
but  as  the  lens  I)'  E'  is  considerably  smaller  than  A 
C,  only  as  many  rays  can  enter  it  as  are  contained  in 
the  angle  I)'  B'  E'.  As  the  rays  contained  in  the 
angle  ABC  and  D'  B'  E'  are  carried  through  the 
two  lenses,  which  are  supposed  to  be  of  the  same 
magnifying  power,  the  image -formed  by  A  C  will  be 
considerably  brighter  than  that  formed  by  D'  E',  and 
will  therefore  show  more  of  its  structure,  as  will  be 
shown  hereafter. 


33 

While  this  is  not  a  theoretically  correct  explanation 
of  angular  aperture,  it  will  serve  the  purpose  of  show- 
ing its  effect.  As  increase  in  aperture  means  increase 
of  resolving  power  and  it  will  be  seen  that  every  degree 
thus  added  increases  the  effectiveness  of  the  objec- 
tive. It  is  in  this  direction  that  modern  advance  has 
been  signalized.  Whatever  views  rrray  be  held  on  the 
advisability  of  narrow  or  wide  angle  objectives,  the 
optical  standard  of  excellence  depends  particularly 
on  this  quality. 

Objectives  of  the  same  angular  aperture,  but  of  dif- 
ferent magnifying  power  (within  ordinary  limits),  will 
show  the  object  equally  well,  provided  they  are  other- 
wise of  the  same  quality,  and  it  is  also  true  that  in 
objectives  of  the  same  power  but  unequal  angular 
aperture,  the  one  of  wider  angle  will  show  an  object 
more  brilliantly  than  the  other,  and,  if  the  difference 
be  considerable,  will  show  structure  of  which  no  trace 
can  be  found  with  the  narrow  angle.  These  are  facts 
which  are  based  upon  natural  laws,  but  there  are 
other  conditions  to  be  considered  in  connection  with 
them,  which  will  be  treated  hereafter. 

It  very  often  happens  that  objectives  from  different 
makers,  but  of  the  same  angle,  show  a  considerable 
variation  ;  this  does  not  prove  that  the  above  princi- 
ple is  wrong,  but  is  evidence  that  greater  care  or  skill 
has  been  bestowed  on  one  than  on  the  other. 

By  arranging  an  objective  with  an  immersion  front, 
its  angular  aperture  may  be  considerably  increased 
over  that  of  a  dry  front,  and  this  explains  why  better 


34 

results  may  be  obtained  with  the  former  than  with  the 
latter. 

Prof.  Abbe  has  introduced  a  new  mode  of  deter- 
mining'and  naming  aperture.  He  calls  it  Numerical 
Aperture,  and  this  expression  is  now  generally 
adopted.  Thus,  1.0  of  numerical  aperture  is  equal 
to  180  degrees  in  air  or  about  82  degrees  in  homo- 
geneous immersion,  and  has  a  direct  relative  value  to 
the  resolving  power  of  the  objective.  A  complete 
aperture  table  may  be  found  in  the  Proceedings  of 
the  Royal  Microscopical  Journal  and  larger  works  on 
the  microscope. 

Achromatism. — As  has  been  stated  before,  when 
single  lenses  are  made  to  give  a  high  magnifying 
power,  the  chromatic  and  spherical  aberrations  pre- 
vent corresponding  advantages  ;  and  as  the  objective 
gives  the  image  which  is  magnified  by  the  eye-piece, 
it  is  evident  that  if  they  exist  in  it,  they  are  increased 
by  the  ocular,  and  that  especial  care  must  be  given  to 
exclude  all  faults  as  much  as  possible  from  it.  Even 
with  the  use  of  flint  glass,  it  is  impossible  to  free  the 
objective  entirely  from  color  ;  there  will  remain  a 
residue  of  green  and  purple,  and  these  colors  will 
fringe  the  object.  These  are  called  the  secondary  spec- 
trum, and  their  presence  in  an  objective  is  usually 
evidence  of  the  highest  correction. 

The  amount  of  color  in  an  objective  depends  some- 
what upon  the  power  of  the  eye-piece,  and  becomes 
more  visible  as  a  higher  power  is  used.  -Color  out- 
side of  the  secondary  spectrum  is  not  always  preju- 


35 

dicial  to  an  objective  ;  for,  if  in  two,  one  shows  the 
structure  of  an  object  with  a  slight  amount  of  it,  the 
other  does  not  show  the  structure  but  gives  a  nearly 
colorless  image,  it  goes  without  argument  to  say  that 
the  first  gives  the  best  results  and  is  therefore  pre- 
ferable. 

Within  recent  years  new  kinds  of  glass  have  been 
found  by  means  of  which  the  amount  of  remaining 
color  has  been  reduced,  and  objectives  constructed 
with  it  are  called  apochromatic. 

If  on  increasing  the  distance  between  the  objec- 
tive and  object  the  latter  shows  a  marked  blue  color, 
and  when  the  distance  is  decreased  a  yellow-red 
color,  the  objective  is  chromatically  under-corrected j 
if,  however,  the  conditions  are  reversed,  if  the  object 
shows  a  yellow  color  when  the  distance  between  it 
and  the  objective  is  increased,  and  blue  when 
decreased,  it  is  over-corrected. 

Resolving  Power. — This  is  the  quality  in  an 
objective  by  which  we  are  enabled  to  see  the  intricate 
structure  and  finer  details  in  an  object.  It  depends 
upon  the  amount  of  angular  aperture,  the  correction 
of  the  chromatic  and  spherical  aberrations,  and  of 
course  upon  the  perfection  of  the  mechanical  work. 
The  power  of  resolving  in  an  objective  is  indicative 
of  the  perfection  of  the  microscope,  for  it  is  almost 
entirely  dependent  upon  it  for  its  quality. 

When  an  objective  is  said  to  resolve  a  structure  or 
a  certain  number  of  lines,  it  means  that  it  shows 
them  under  certain  conditions  of  light.  It  may 


36 

resolve  easily  or  only  glimpse  them — the  latter  when 
they  are  hardly  to  be  distinguished.  The  angular 
aperture  of  an  objective  indicates  the  resolving 
power,  and  the  theoretical  capacity  of  every  degree 
has  been  mathematically  determined.  However,  this 
standard  is  only  reached  approximately  and  to  a  vary- 
ing extent.  It  is  not  by  any  means  said  that  every 
objective  of  a  certain  angular  aperture  will  have  a  cor- 
responding resolving  power  ;  it  is  at  this  point  that 
the  acute  accuracy  of  work  and  superior  judgment  of 
the  optician  in  making  proper  corrections  will  invari- 
ably give  the  best  results. 

Minute  structure  such  as  bacter-ia  can  only  be  seen 
by  objectives  having  high  numerical  aperture,  and 
these  are  absolutely  necessary  in  modern  investiga- 
tions. The  many  recent  discoveries  can  only  be 
attributed  to  the  increased  resolving  power. 

It  is  an  error  to  suppose  that  the  resolving  power 
may  be  improved  by  merely  increasing  the  magnify- 
ing power.  It  is  an  invariable  quality  of  an  objective 
and  has  a  fixed  limit.  The  extent  to  which  it  may  be 
approached  depends  upon  the  nicety  of  manipulation, 
but  no  amount  of  increase  in  magnifying  power  by 
the  eye-piece  or  any  other  means  will  carry  it  beyond 
it  ;  on  the  contrary,  it  will  lose  in  this  respect  if  car- 
ried beyond  a  certain  point. 

Flatness  of  Field. — Theyfc/;/in  a  microscope  is 
that  portion  which  is  observed  in  the  eye-piece,  and 
its  flatness  may  be  observed  when  focused  on  a  flat 
object — preferably  a  micrometer.  It  is  said  to  be  flat 


37 


when  all  portions  of  the  object  are  seen  over  the 
entire  field  at  once  without  further  focusing.  When 
not  flat,  it  will  be  found  that  as 
the  image  approaches  the  edge 
of  the  field  it  becomes  more  and 
more  indistinct,  and  that  the 
objective  must  be  correspond- 
ingly adjusted  ;  in  many  cases 
it  remains  indistinct  or  blurred, 
and  this  may  be  considered  the 
Fig.  ii.  most  serious  fault.  In  the  case 

of  looking  at  straight  parallel  lines,  such  as  in  a 
micrometer,  they  will  appear  to  become  more  curved 
as  they  near  the  edge,  as  shown  in  Fig.  11. 

Flatness  of  field  mainly  depends  upon  the  correc- 
tion of  the  spherical  aberrations,  and  as  under  the 
best  conditions  the  latter  cannot  be  entirely  eliminated, 
it  is  impossible  to  attain  absolute  flatness,  except  with 
eye-pieces  especially  made  for  this  purpose.  It  may, 
however,  also  be  due  to  a  faulty  eye-piece  ;  in  this 
case  it  can  fairly  be  determined,  by  observing 
whether  it  shows  equally  in  different  objectives.  With 
beginners,  especially,  it  is  usually  most  complained 
of,  owing  probably  to  the  fact  that  it  is  the  most  easily 
noticable.  It  is  a  desirable  quality  and  indicates  to  a 
considerable  degree  the  quality  of  objectives.  While 
it  is  impossible  to  obtain  absolute  flatness,  the  opti- 
cian's effort  is  to  obtain  the  nearest  approach  to  it.  As 
a  quality  in  itself,  without  regard  to  resolving  power,  it 
is  most  easily  obtainable,  but  in  connection  with  this 
quality  it  becomes  difficult  to  acquire. 


38 

Penetration. — This  is  the  quality  which  enables 
us  to  look  into  an  object— to  observe  different  planes 
at  one  time.  In  the  mind  of  the  writer,  it  is  of  no 
special  importance,  or  at  any  rate  not  as  much  as  is 
claimed  for  it,  and  if  desired  is  easily  attained.  It 
depends  upon  magnifying  power  and  angular  aper- 
ture, and  decreases  with  the  increase  of  either  of 
these.  Objectives  are  generally  not  constructed  with 
any  reference  to  it ;  it  is  a  natural  consequence  of 
certain  conditions. 

Penetration  and  resolving  power  are  antagonistic, 
or  at  any  rate  in  an  inverse  ratio,  and  can  only  be 
combined  to  a  certain  extent.  In  two  objectives 
of  the  same  power  and  aperture,  one  cannot  have 
penetration  as  a  special  feature  and  the  other  resolv- 
ing power  ;  they  will  be  almost  similar  in  these  quali- 
ties, provided  that  they  are  similarly  corrected.  How- 
ever, if  they  are  not  similar  in  their  angular  aperture 
the  one  of  small  aperture  will  have  more  penetration 
than  the  other.  In  objectives  of  the  same  angle  but 
different  power,  the  one  of  low  power  will  have  in 
itself  more  penetration  ;  it  will  be  similar  in  its  action 
to  the  eye,  which,  when  an  object  is  close  to  it,  can 
distinguish  but  one  portion  of  it  distinctly,  while,  as 
its  distance  to  the  eye  is  increased,  can  distinguish 
various  parts  of  it  lying  at  different  distances,  and 
will  finally  see  other  objects  outside  of  it.  By  look- 
ing at  an  object  at  5  feet  distance,  only  this  can  be 
seen  plainly;  but,  at  10  feet,  others  quite  a  distance 
in  front  or  back  of  it  can  be  seen  as  well. 


39 

Working  Distance.  — This  term,  strictly  con- 
sidered, is  an  invariable  quality  of  the  objective,  and 
is  the  distance  between  the  front  lens  in  the  objective 
and  an  uncovered  object,  when  the  objective  is  in 
focus  and  is  corrected  for  that  object.  All  objectives 
require  a  certain  amount  of  projecting  metal  to  pro- 
tect the  front  lens,  and  this  with  a  certain  thickness 
of  the  cover-glass  lessens  it.  In  objectives  with  fixed 
mountings  this  may  be,  and  with  thick  cover-glasses 
is  considerable.  As  it  is  comparatively  unimportant, 
however,  for  the  working  microscopist  to  know  the 
working  distance  per  se  of  his  objectives,  but  of  con- 
siderable moment  to  know  what  the  actual  space 
between  the  objective  and  cover  glass  is,  it  would  be 
well,  in  the  mind  of  the  writer,  to  express  it  as  avail- 
able working  distance. 

In  objectives  of  low  and  medium  power,  it  is  of 
little  consideration  ;  but  where  it  must  be  expressed 
in  -j-^.  or  TTrV  IT  inch,  it  becomes  a  matter  of  importance. 

Working  distance  is  spoken  of  as  being  long  or 
short,  and  varies  not  so  much  with  the  power  as  with 
the  angular  aperture  ;  generally  the  working  distance 
decreases  with  the  increase  in  angular  aperture,  and 
becomes  greater  as  the  aperture  becomes  smaller  ;  it 
it  was  for  a  long  time  considered  that  these  two  prop- 
erties varied  according  to  a  fixed  rule,  but  this  at  the 
present  time  is  not  considered  to  be  the  case.  While 
in  objectives  of  the  same  aperture  it  may  vary  con- 
siderably, it  may  in  others  of  different  aperture  be  so 
that  the  higher  one  may  have  the  greater  working 


40 

distance.  The  skill  of  the  optician  must  in  a  consid- 
erable manner  determine  the  amount  of  it. 

It  will  be  seen  from  the  above  that  working  dis. 
tance  stands  in  no  direct  relation  to  the  focal  distance 
of  the  objective,  neither  to  its  nomenclature  or  rat- 
ing, and,  it  may  be  added,  that  it  is  never  as  great  as 
the  focal  distance  of  a  single  lens  of  the  same  magni- 
fying power. 

As  may  be  imagined,  there  are  a  variety  of  opin- 
ions as  to  what  constitutes  long  or  short  working  dis- 
tance in  a  certain  objective.  No  definite  rule  can  be 
laid  down  for  this,  as  it  is  conditioned,  by  the  skill 
and  requirements  of  the  manipulator.  Although  it  is 
an  important  factor,  the  idea  that  it  should  in  all 
cases  be  as  great  as  possible,  is  erroneous,  for,  while 
it  may  be  true  in  a  dry  objective,  it  may  be  the  cause 
of  annoyance  in  one  with  immersion.  On  several 
occasions  it  occurred  in  the  experience  of  the  writer 
that  after  an  objective  had  been  completed,  it  was 
found  that  its  working  distance  was  so  large  that  the 
immersion  fluid  would  run  out  from  between  the 
objective  and  the  cover-glass  when  the  instrument 
was  inclined,  and  it  was  necessary  to  change  the 
objective  with  a  view  to  decreasing  its  working  dis- 
tance, in  order  to  allow  its  convenient  use. 

Magnifying  Power. — This  is  a  question,  of  vital 
importance  in  a  microscope,  not  so  much  as  a  quality 
for  itself,  as  in  connection  with  the  resolving  power. 
The  inquiry  should  not  be  simply  how  many  diame- 
ters an  instrument  will  magnify,  but  what  the  precision 


41 

and  extent  of  its  definitions  are  under  a  certain  mag- 
nifying power.  If  a  high  magnifying  power  is  all  that 
is  desired,  this  may  be  obtained  to  an  almost  unlim- 
ited extent  by  means  of  simple  lenses  which  may  be 
procured  at  a  small  pecuniary  outlay  ;  but  these  do 
not  give  a  distinct  image  nor  do  they  make  structure 
visible,  which,  be  it  remembered,  is  the  purpose  of 
the  microscope  to  do. 

The  normal  eye  can  distinguish  from  200  to  250 
lines  to  the  inch,  and  in  a  microscope  such  magnify- 
ing power  should  be  used,  which  will  apparently  bring 
the  structure  which  is  sought  after  at  least  up  to  this 
figure.  In  illustration  take  a  \  inch  objective  of  98 
degrees  and  a  \\  inch  eye-piece.  An  objective  of  this 
kind  properly  corrected,  resolves  pleurosigma  angiila- 
tum,  in  which  the  average  lines  are  00,000  to  the 
inch.  With  the  above  eye-piece  it  is  utterly  impossi- 
ble to  see  them,  while  if  it  is  replaced  by  a  |  inch  or 
|  inch,  they  can  easily  be  distinguished.  This  is  not 
owing  to  any  peculiar  quality  of  the  eye-piece,  but 
merely  to  the  fact  that  by  increasing  the  magnifying 
power,  the  dimensions  of  the  object  have  been 
increased  to  such  an  extent  that  the  lines  have  appar- 
ently been  separated  and  become  visible  to  the  eye. 

Beginners  as  a  rule  are  apt  to  use  too  much  magni- 
fying power  or  amplification,  and  often  attempt  to 
view  a  large  surface  with  an  objective  which  will  show 
but  a  small  part  of  it.  It  must  not  be  forgotten  that 
the  apparent  field  of  view  is  decreased  as  higher 
powers  are  used,  and  that  a  low  power  will  give  a 


42 

better  impression  of  a  large,  coarse  object  and  its 
relative  parts,  not  only  because  it  makes  a  larger  sur- 
face visible,  but  because  it  has  more  penetration. 

In  objectives  of  the  same  power,  but  of  different 
angular  aperture,  the  magnifying  power  and  field  will 
always  be  the  same. 

The  following  table  which  has  been  compiled  will 
probably  be  of  assistance  to  the  beginner.  After  he 
has  become  better  acquainted  with  his  instrument  his 
judgment  will  dictate  to  him  what  to  do. 

A  power  of  25  diameters  will  show  a  surface  of 
about  J  inch  diameter. 

A  power  of  50  diameters  will  show  a  surface  of 
about  TL  inch  diameter. 

A  power  of  100  diameters  will  show  a  surface  of 
about  gV  inch  diameter. 

A  power  of  500  diameters  will  show  a  surface  of 
about  yi^  inch  diameter. 

A  power  of  1000  diameters  will  show  a  surface  of 
about  TjJ-Q-  inch  diameter. 

This  table  is  approximately  correct  with  a  Huy- 
ghenian  eye-piece  ;  with  a  Periscopic  almost  double 
the  amount  of  such  surface  will  be  shown. 

Magnifying  power  may  be  obtained  by  the  eye- 
piece or  objective  andthe  desirability  of  using  one  or 
the  other  for  this  purpose  was  for  many  years  a  mat- 
ter of  spirited  discussion,  but  it  is  now  generally  con- 
ceded that  increased  power  should  be  obtained  by 
increasing  the  power  of  objectives  and  not  go  beyond 
the  equivalent  power  of  1  inch  in  the  eye-pieces. 

Objectives  of  the  same  angular  aperture,  but  of 
different  power,  will  give  identical  results  by  bringing 
them  up  to  the  same  magnifying  power,  unless  the 


43 

difference  is  considerable.  In  both  objectives  and 
eye-pieces  the  lenses  decrease  in  size  with  the  increase 
in  power  and  consequently  give  less  light ;  and  while 
this  one  objection  exists  in  the  objective  an  addi- 
tional one  occurs  in  the  eye-piece,  in  that  the  eye 
must  be  brought  closer  to  the  eye-lens  and  must  be 
kept  more  strictly  in  the  optical  axis,  which  at  a  long 
sitting  becomes  fatiguing. 

Between  the  1  inch  and  2  inch  the  choice  should  be 
determined  by  requirements  and  individual  prefer- 
ence. All  responsible  manufacturers  and  dealers  make 
up  such  outfits  of  stands,  objectives  and  eye-pieces, 
which  experience  has  taught  them  are  most  generally 
useful. 

It  is  a  safe  rule  to  follow  in  all  work  on  recognized 
forms  ( objects  of  which  the  structure  is  known  )  not 
to  use  a  higher  power  than  is  necessary  to  properly 
study  them. 

Eye-Piece — Huyghenian. — This  is  now  in  gen- 
eral use,  and  consists  of  two  plano-convex  lenses.     It 
^~ — -\         r — ~^>       receives     its     name     from     the 
v/    —    \J          inventor,  who  first  applied  it  to 
the  telescope.  The  eye-lens  is  the 
small  lens  nearest  the  eye,  and 
the  fie 'Id- lens,  or  collective  as  it  is 
also  called,  is  the  large  one  near- 
est the  objective.     A  diaphragm 
is    placed    between    them,    and 
gives    a    sharply   defined    field. 
This  eye-piece  is  also  called  neg- 
ative,  as  its  focal  point  is  between 


44 

the  two  lenses  (at  the  diaphragm)  in  contradistinction 
to  a  positive,  in  which  the  focal  point  is  outside  of 
and  below  the  field-lens. 

The  Continental  eye-piece  is  also  a  Huyghenian, 
although  it  is  mounted  in  a  straight  tube,  in  place  of 
the  mounting,  with  the  neck  as  shown  in  cut.  The 
American  Society  of  Microscopists  has  recently 
recommended  that  eye-pieces  be  made  par-focal,  that 
is,  that  the  equivalent  foci  coincide  and  that  the  par- 
focal  plane  correspond  with  the  upper  end  of  the 
tube.  This  is  an  excellent  plan,  as  the  focus  on  the 
object  is  maintained  whatever  may  be  the  change  in 
eye-pieces. 

Solid  Eye-Piece. — This  was  the  invention  of  the 
late  R.  B.  Tolles,  and  also  belongs  to  the  class  of 
negative  eye-pieces.  It  is  called  solid 
from  the  fact  that  instead  of  being 
composed  of  two  lenses,  it  consists 
of  one  piece  of  glass,  which  is  cut  to 
a  cylindrical  form,  and  on  the  ends 
of  which  the  proper  curvatures  are 
ground  ;  the  diaphragm  is  made  by  Fig.  13. 

cutting  a  circular  groove  into  the  glass  at  the  proper 
distance  between  the  two  surfaces,  which  is  then  filled 
up  with  an  opaque  pigment. 

These  eye-pieces  are  only  made  in  high  powers,  as 
optical  glass  is  usually  not  of  sufficient  homogenuity 
to  make  low  powers,  and  their  cost  would  be  too  con- 
siderable, without  a  corresponding  advantage.  For 
high  powers  they  are  superior  to  the  Huyghenian,  in 


45 

that  they  give  a  better  illuminated  field,  as  there  is 
less  loss  of  light  by  absorption  through  the  glass  than 
by  refraction  at  the  two  additional  surfaces  of  the  eye- 
lens  and  field-lens  in  the  Huyghenian. 

Periscopic  Eye-Piece. — This  consists  of  a  triple 
eye-lens  and  single  field-lens.  Its  predominant  fea- 
ture is  a  very  large  and  flat  field, 
with  almost  all  objectives.  In 
this  respect  it  has  a  considerable 
advantage  over  the  Huyghenian 
and  Solid.  It  is  positive  and 
therefore  well  adaped  for  micro- 
meter work,  as  it  is  focused  like 
a  magnifier,  and  its  magnifying 
power  remains  constant,  while 
with  the  Huyghenian  it  is  vari- 
Fig.  14.  able,  from  the  fact  that  the  eye- 

lens  alone  is  focused,  thus  varying  its  distance  from 
the  field-lens,  and  consequently  the  magnifying  power. 
Nomenclature. — The  rating  of  eye-pieces  was 
formerly,  and  is  to  a  considerable  extent  to-day,  by 
letters.  This  method,  however,  is  arbitrary,  as  the 
letters  of  different  makers  have  a  totally  different  sig- 
nificance, so  that  nothing  like  a  standard  exists.  This 
fact  induced  the  American  Society  of  Microscopists 
to  endeavor  to  establish  a  universal  method,  and  after 
the  matter  had  been  given  careful  attention  for  sev- 
eral successive  years,  it  finally  adoped  the  method 
which  rates  them  according  to  their  magnifying 
powers,  the  same  as  that  which  has  been  used  in 
objectives.  .This  gives  an  approximate  idea  of  the 


46 

magnifying  powers  ;  thus,  an  eye-piece  marked  1  inch 
or  by  a  letter  signifying  the  same,  shows  that  it  mag- 
nifies about  10  diameters  ;  one  of  |  inch,  20  diame- 
ters, and  so  on. 

Flatness  Of  Field.— Although  this  depends 
mainly  upon  the  objective,  the  absence  of  it  may  be 
owing  to  a  faulty  construction  of  the  eye-piece.  If  it 
is  so  prominent  as  to  be  easily  noticeable,  and  to  the 
same  degree  with  a  number  of  objectives,  it  may  be 
ascribed  to  the  eye-piece.  It  must,  however,  be  remem- 
bered that  an  absolutely  flat  field  has  not  yet  been 
obtained  ;  it  may  be  closely  approached  by  decreas- 
ing the  diameter  of  field  to  less  than  its  normal  size. 

Size  of  Field. — Quite  a  general  but  errorneous 
idea  prevails  that  the  size  of  the  tube  has  an  influence 
on  the  size  of  the  field.  Except  in  eye-pieces  of  very 
low  power,  or  with  tubes  with  smaller  than  usual 
dimensions,  this  is  not  so.  It  must  be  remembered 
that  a  Huyghenian  eye-piece  admits  of  a  definite  size 
of  field,  and  this  is  regulated  by  the  opening  in  the 
diaphragm  ;  the  same  size  of  opening  is  used  in  all  of 
the  same  power,  whether  it  is  an  eye-piece  for  a  large 
or  small  diameter. 

A  misconception  also  exists  as  to  the  definition  of 
field.  Such  inquiries  are  often  made  as  :  "  As  we 
understand  it,  a  wide-angle  objective  gives  a  larger 
field  ? "  but  it  does  nothing  of  the  kind.  The  angular 
aperture  has  no  bearing  whatever  on  the  size  of 
the  field,  The  field  of  view,  or  that  which  is  shown 
of  the  object's  surface,  is  determined  by  the  power  of 
the  objective  and  eye-piece. 


REQUISITES   FOR  WORK. 


It  is  the  intention  to  make  such  recommendations 
in  this  chapter  which,  if  not  absolutely  necessary,  will 
be  found  convenient  and  will  aid  in  facilitating  work. 

One  of  the  first  requisites  for  the  proper  use  of  the 
microscope,  is  a  thorough  knowledge  of  its  parts  and 
an  acquaintance  with  the  optical  principles  involved. 
For  this  purpose  the  writer  earnestly  requests  a 
perusal  of  the  preceding  pages,  and  is  convinced  that 
in  cases  where  no  previous  knowledge  of  the  instru- 
ment has  existed,  work  will  be  .done  with  far  more 
ease,  in  much  less  time,  and  with  a  greater  degree  of 
satisfaction.  Ignorance  of  the  instrument's  capacity 
may  lead  to  an  idea  that  it  is  inferior  and  thus  be  the 
means  of  its  final  abandonment ;  and  in  place  of  the 
anticipated  pleasure  there  may  arise  a  feeling  of  bit- 
terness and  disappointment  for  all  future  with  every- 
thing connected  with  it.  There  are  innumerable  cases 
of  this  kind  and  they  have  induced  a  belief  that  it  is 
difficult  to  acquire  a  practical  manipulation  of  the 
microscope,  whereas  such  is  not  the  case  when  a  lim- 
ited time,  properly  applied,  is  devoted  to  it. 

Working"  Table. — A  firm  table  should  be  used, 
preferably  one  with  three  legs,  as  this  will  always  be 
firm  no  matter  how  uneven  the  floor  is,  and  if  it  can 


48 

be  arranged,  should  be  devoted  to  this  purpose  only. 
One  with  a  round  or  square  top  of  three  feet  provides 
ample  room.  Although  not  necessary,  a  table  with  a 
revolving  top,  provided  with  clamp,  is  very  conven- 
ient, as  with  this  two  or  more  persons  may  make 
observations  without  changing  their  seats. 

A  very  neat  arrangement  for  a  table-top  is  that  sug- 
gested and  used  by  Dr.  J.  E.  Reeves.  He  places  upon 
an  ordinary  table  three  or  four  thicknesses  of  white 
paper  and  upon  these  a  plate  of  polished  glass  as 
large  as  the  top  ;  this  can  be  procured  of  almost  any 
glazier  at  a  low  price.  It  is  pleasant  to  work  upon 
and  will  not  soil. 

As  in  almost  all  cities  there  is  more  or  less  contin- 
ual vibration  from  wagons  upon  the  paved  streets,  the 
writer  suggests  an  effectual  remedy.  Take  a  thin 
board,  say  half  an  inch  thick,  of  a  sufficient  size  to 
receive  the  microscope  ;  fasten  on  the  upper  side  at 
two  opposite  ends,  cleats  of  1  inch  square  and  coun- 
ter-sink into  these  through  the  board  four  spiral 
springs  of  such  tension  that  when  they  bear  the 
weight  of  the  instrument,  the  bottom  of  the  board 
will  be  about  \  inch  from  the  table. 

Have  the  working  table  provided  with  drawers  and 
arrange  receptacles  for  the  accessories,  secure  from 
dust,  but  at  a  convenient  point  to  reach.  When  the 
instrument  is  not  in  use  put  it  into  its  case  or  cover 
it  in  a  manner  so  that  it  shall  be  free  from  dust.  For 
this  purpose  a  large  bell  glass  is  best. 


49 

Room. — If  possible  a  room  should  be  selected  fac- 
ing the  north,  as  the  light  in  this  direction  is  most 
constant.  It  will  prove  a  great  saving  of  time  if  all 
or  a  portion  of  it  can  be  permanently  arranged  to 
receive  the  entire  working  outfit.  It  should  also  be 
chosen  with  a  view  to  its  being  free  from  disturbance. 

Light — As  stated,  the  light  from  the  northern  sky 
is  most  desirable,  and  that  from  a  white  cloud  is  pre- 
ferable to  that  from  a  blue  sky.  On  account  of  its 
intensity,  direct  sunlight  should  seldom  be  used  ;  but 
if  modified  by  a  white  curtain  or  reflected  from  a 
white  wall  it  is  excellent. 

For  lamp  light  an  ordinary  flat  wick  kerosene  or 
student  lamp  is  well  adapted.  The  Hitchcock  lamp, 
from  its  better  combustion  is  still  better,  as  its  color 
more  nearly  approaches  white.  The  ideal  artificial 
light  is  that  from  an  electric  light.  Gas  light  is  not 
desirable  as  it  is  seldom  sufficiently  steady. 

Position  of  Light. — The  relation  of  the  micro- 
scope to  the  source  of  light  is  principally  a  matter  of 
personal  convenience.  With  daylight  it  makes  little 
difference  whether  it  is  at  the  front  or  side  of  the 
instrument,  although  the  writer  prefers  it  at  the  front, 
as  the  manipulation  of  the  object  does  not  obstruct 
it  ;  but  the  lamp  should  be  placed  at  the  right  or  left 
side  within,  easy  reach  of  the  hand  for  the  purpose  of 
controlling  it.  The  writer  suggests  that  the  beginner 
make  it  a  habit  at  the  outset  to  place  it  on  the  side  of 
the  instrument  opposite  to  the  unoccupied  eye,  as  the 
tube  then  places  the  latter  in  the  shadow. 


50 

Which  Eye  to  Use. — In  a  binocular  instrument 
both  eyes  are  used,  but  in  a  monocular  only  one  is 
used,  and  it  depends  upon  a  trial  which  is  best  suited. 
A  large  proportion  of  persons  are  afflicted  with  astig- 
matism, often  without  knowing  it,  and  when  this 
exists  it  may  be  in  one  eye  or  when  in  both,  may  be 
to  a  greater  extent  in  one  than  in  the  other.  Its 
presence  may  prevent  the  eye  from  observing  fine 
detail ;  but  whichever  eye  is  found  to  be  best  suited 
should  be  used.  When  both  eyes  are  alike  it  is  some- 
times advisable  to  change  from  one  to  the  other. 

It  should  be  made  a  habit  at  the  outset  and  strictly 
adhered  to,  to  keep  both  eyes  open.-  A  little  difficulty 
may  be  found  to  do  this,  as  the  eye  which  is  free  will 
probably  observe  the  objects  upon  the  table  ;  but  as 
soon  as  the  mind  becomes  fixed  upon  what  it  sees  in 
the  microscope,  this  impression  disappears.  After  a 
time  it  will  be  found  to  require  no  exertion  and  will 
certainly  add  to  the  ease  and  comfort  of  the  manip- 
ulator while  working. 

The  Ward  Eye  Shade,  Fig.  15,  will  prove  of  as- 
sistance in  acquir- 
ing the  above  men- 
tioned habit,  and 
ebsides  this,  effect- 
ually excludes  the 

Fig.  15.  light  from  the  eyes. 

It  is  made  of  hard  rubber  and  is  attached  to  the  tube 
of  the  microscope. 


51 

Order. — Among  the  requisites  for  successfully 
prosecuting  work  with  the  microscope  are  a  strict 
observance  of  the  instructions,  even  if  they  appear 
superfluous,  a  systematic  way  of  doing  work,  and 
cleanliness.  Have  a  place  for  every  article  which 
is  required,  so  that  the  hand  may  immediately  be 
placed  upon  it ;  after  it  has  been  used  clean  it  before 
putting  it  aside  ;  keep  strange  hands  from  your 
apparatus  unless  you  are  assured  that  a  knowledge  of 
its  manipulation  exists. 

Material. — Although  the  purpose  of  this  manual 
is  to  be  a  guide  to  the  intelligent  use  of  the  micro- 
scope and  not  the  preparation  or  preservation  of 
objects,  it  may  not  be  out  of  place  here  to  enumerate 
what  every  owner  of  an  instrument  should  have  at 
the  outset.  The  first  should  be  a  book  on  objects 
giving  proper  instruction  on  their  preservation. 
There  are  many  of  these,  and  all  of  them  good.  Next 
in  order,  slides,  covers  and  labels  are  necessary.  As 
covers  are  easily  broken  in  cleaning,  a  larger  propor- 
tion of  them  will  be  necessary. 

A  cabinet  for  slides,  a  large  variety  of  which  may 
be  selected  from,  will  aid  in  starting  work  in  a  sys- 
tematic manner.  Forceps  and  a  small  pipette  are 
indispensible.  For  preserving  objects,  Canada  bal- 
sam  or  damar  should  be  purchased,  while  the  other 
necessary  material  which  may  be  gleaned  from  the 
instruction  book  is  easily  obtainable. 

When  it  is  intended  to  do  section-cutting  a  good 
mechanical  microtome,  not  necessarily  expensive, 
should  be  obtained  at  the  outset. 


HOW   TO   WORK. 


To  Set  Up  the  Instrument.— Draw  the  instru- 
ment from  the  case  by  grasping  the  base,  and  free  it 
from  dust.  If  it  has  draw  tube,  bring  it  to  its  stand- 
ard length,  which  is  indicated  by  a  ring,  by  as  little 
of  a  screw  motion  as  possible  ;  if  the  draw  tube  is 
highly  polished,  its  surface  will  be  best  retained  by 
observing  the  above  precaution.  See  that  the  mirror 
and  largest  aperture  of  the  diaphragm  are  in  a  central 
position — in  the  optical  axis.  After  being  convinced 
that  the  eye  pieces  are  clean,  place  one  into  the  tube  ; 
then  remove  the  objectives  from  their  cases  and  after 
first  having  increased  the  distance  between  the  stage 
and  tube  by  means  of  the  milled  heads  of  the  pinion, 
attach  the  lowest  power  to  the  nose-piece,  by  using 
both  hands,  being  careful  that  it  is  as  near  as  possible 
in  the  optical  axis  while  screwing  it  on.  Then  incline 
the  body  by  placing  the  left  hand  upon  the  base  and 
drawing  with  the  right  hand  upon  the  arm  ;  be  care- 
ful not  to  pull  on  the  tube,  as  it  may  prove  too  heavy 
a  strain  upon  this  or  the  fittings.  Incline  the  body 
of  the  microscope  until  the  eye  piece  about  reaches 
the  level  of  the  eye,  so  that  when  an  observation  is 
made  the  position  is  as  comfortable  as  possible  ;  the 


53 

neck  should  not  be  strained,  neither  should  the  chest 
be  compressed.  Next  place  the  slide  with  a  transpar- 
ent object  upon  the  stage,  by  sliding  it  under  the  spring 
dips  and  get  it  as  near  as  possible  in  the  center  of  the 
opening  ;  for  an  object  anything  near  at  hand,  such 
as  a  piece  of  printed  paper  or  cotton  fibres  will  do. 
Watching  the  slide,  adjust  the  mirror  until  it  is  seen 
that  the  light  strikes  the  object ;  incline  the  head  to 
the  level  of  the  stage,  and  observing  the  objective, 
rack  it  down  to  within  i  inch  of  the  object.  Again 
placing  the  eye  at  the  eye- piece,  reverse  the  motion 
of  the  milled  heads  and  observing  the  field  continue 
the  upward  motion  of  the  body  until  the  image  of 
the  object  appears  in  view. 

Centering  Stage. — If  the  microscope  has  a 
revolving  stage,  turn  this  to  see  whether  the  object 
or  portion  of  it  lying  in  the  center  of  the  field, 
remains  in  the  optical  axis.  It  was  true  when  the 
instrument  was  shipped,  but  may  have  changed  dur- 
ing transportation.  If  not  centered,  loosen  the  screw- 
holding  it  to  the  arm  by  means  of  the  steel  pin,  just 
sufficiently  that  by  the  exertion  of  a  little  pressure  it 
can  be  moved.  After  having  observed  first  which  por- 
tion of  the  object  remains  stationary  during  its  revo- 
lution (this  evidently  is  its  center)  move  the  stage  so 
that  this  point  will  be  in  the  center  of  the  field,  and 
then  tighten  the  screw.  If  the  point  lies  outside  of 
the  limits  of  the  field,  its  direction  can  be  noted  and 
the  stage  moved  accordingly.  Where  centering  screws 
are  provided  in  the  instrument,  this  is  a  simple  mat- 
ter. 


54 

If  the  coarse  adjustment  does  not  prove  sensitive 
enough  to  focus  easily,  adjust  by  the  fine  adjustment 
by  taking  the  head  of  the  micrometer  screw  between 
the  thumb  and  first  finger  and  move  toward  the  right 
or  left  as  may  be  necessary. 

It  may  here  be  said  in  passing  that  the  rack  and 
pinion  should  be  so  well  fitted  that  they  should  per- 
mit the  adjustment  of  low  power  objectives  with  the 
greatest  ease,  and  should  work  without  the  slightest  lost 
motion  with  a  \  or  \  inch  objective.  This  point  is  the 
criterion  of  workmanship  in  an  instrument,  and  if  it 
is  found  to  have  the  least  back-lash,  or  is  not  perfectly 
smooth,  it  may  safely  be  assumed  that  the  instrument 
is  of  inferior  workmanship. 

If  the  fine  adjustment  does  not  act,  the  screw  has 
either  come  to  its  stop,  or  has  "run  out,"  and  must 
be  brought  into  action  again  ;  the  range  of  movement 
in  almost  all  fine  adjustments  is  quite  short  and  con- 
stant care  must  be  taken  to  keep  it  at  about  a  medium 
point.  If  the  object  is  found  not  to  give  a  full  view 
or  is  not  in  the  center  of  the  field,  it  must  be  moved 
on  the  stage,  but  it  must  be  remembered  that  a  move- 
ment in  one  direction  causes  an  apparent  opposite 
movement  in  the  field.  At  first  this  movement  will  be 
in  jerks,  but  after  a  little  practice  the  necessary  sen- 
sitiveness of  touch  is  acquired  to  give  it  more  steadi- 
ness. 

Illumination.  —  It  should  now  be  observed 
whether  the  field  is  equally  illuminated.  Too 
much  stress  cannot  be  laid  on  this  point, 


55 

as  it  is  one  which  is  easily  overlooked  and 
is  often  the  cause  of  considerable  mischief. 
If  the  light  comes  through  a  window  a  well  defined 
image  of  the  sash  is  reflected  by  the  mirror,  and  with 
a  lower  power  objective  this  can  easily  be  seen  ; 
unless  the  mirror  is  correctly  adjusted  the  field  will 
appear  to  be  crossed  by  dark  bands.  In  the  case  of 
lamp  light  the  flame  is  reflected  and  has  a  similar 
effect.  With  high  powers  this  fault  is  not  so  easily 
noticed,  and  for  this  reason  require  the  more  care  ; 
proper  resolution  may  in  this  manner  be  partially  or 
totally  destroyed.  The  remedy  is  either  by  shifting 
the  mirror  or  by  varying  its  distance  from  the  object. 
More  information  on  this  subject  is  given  under  the 
head  of  Sub-stage  Illumination. 

Attaching  High- Power  Objectives. — As  the 

difficulty  of  properly  getting  an  object  or  a  certain 
portion  of  it  in  the  field,  increases  with  the  magnifying 
power,  it  is  a  good  rule  to  use  the  lower  power  objec- 
tive as  a  "  finder;"  after  getting  the  point  to  be  further 
examined  in  the  center  of  the  field,  remove  the  objec- 
tive and  attach  the  higher  power,  and  after  following 
the  procedure  of  focusing  as  with  the  low  power, 
except  that  the  objective  should  be  brought  almost 
in  contact  with  the  cover,  the  point  will  be  seen  in 
the  field  or  will  be  found  to  be  close  to  it. 

This  plan  of  focusing  as  suggested  above  is  always 
a  good  one  to  follow  and  is  observed  by  many  of  the 
most  expert  manipulators.  In  many  cases,  however, 
the  focus  is  obtained  without  this  precaution,  by 


56 

watching  the  field  as  the  objective  is  brought  down 
toward  the  object,  but  is  often  followed  by  disastrous 
results.  Almost  any  person  who  has  used  the  micro- 
scope for  any  length  of  time  is  without  question 
aware  that  valuable  preparations  have  been  destroyed 
in  this  way. 

How  to  Work. — It  is  now  supposed  that  the 
instrument  is  ready  for  work.  To  start,  it  is  well  for 
the  beginner  to  provide  a  few  prepared  specimens,  as 
these  will  help  him  considerably  if  it  is  his  intention, 
as  it  should  be,  to  prepare  them  later  himself.  What- 
ever branch  of  study  he  is  going  to  follow,  a  slide  of 
pleurosigma  angulatum,  dry,  will  be  valuable  to  prac- 
tice upon  and  to  determine  the  quality  of  his  higher 
power  objective.  In  this  latter  respect,  however,  the 
writer  would  advise  the  beginner  to  guard  against 
expressing  an  opinion  too  soon.  He  knows  of  many 
cases  where  the  optician's  claims  were  flatly  denied, 
when  often  a  few  words  of  advice  by  lettter  or  a  few 
minutes  of  intelligent  manipulation  would  resolve  the 
diatom,  and  would  thereafter  do  it  so  easily  that  it 
became  a  wonder  how  it  could  possibly  be  avoided. 

For  a  low  power  objective,  the  proboscis  of  a  blow- 
fly is  probably  the  most  suitable  and  at  the  same  time 
most  interesting  object.  Place  this  upon  the  stage, 
and,  after  getting  it  as  close  as  possible  to  the  center 
of  the  opening  in  it,  focus  by  means  of  the  coarse 
adjustment.  If  only  a  portion  of  it  can  be  seen  and 
if  it  is  desired  to  see  a  larger  surface,  the  length 
of  tube  may  be  contracted  by  means  of  the  draw-tube. 


57 

In  this  case  the  object  will  be  placed  out  of  focus, 
and  another  adjustment  becomes  necessary.  If  a 
higher  power  is  desired,  the  draw-tube  may  be 
extended.  Observe  whether  the  field  is  well  illumi- 
nated, and  if  not,  bear  in  mind  what  has  been  said  about 
properly  adjusting  the  mirror.  If  the  object  appears 
milky  or  the  light  is  so  intense  as  to  be  painful  to  the 
eye,  which  is  of  usual  occurrence  to  the  beginner,  the 
diaphragm  should  be  turned  from  one  aperture  to 
another  until  a  marked  difference  is  seen  ;  or,  the  plane 
mirror  should  be  used.  In  this  connection  it  is  well  to 
state  that  the  above  precautions  should  always  be 
observed  with  low  powers,  unless  the  object  is  thick. 
Now  use  the  micrometer  screw  and  note  carefully  the 
beautiful  structure  which  is  opened  to  view.  After 
sufficient  time  has  been  spent,  upon  this,  the  objec- 
tive may  be  replaced  by  a  higher  power  and  the 
object  by  a  slide  of  P.  angulatum  ;  focus  upon  this, 
being  mindful  of  the  suggestions  previously  given,  and 
do  not  fail  to  observe  what  has  been  said  in  regard 
to  a  well  illuminated  field.  If  lamp  light  from  a  flat 
wick  is  used,  turn  the  edge  of  the  flame  toward  the 
mirror,  and  use  the  concave  side  of  the  latter.  If  the 
diaphragm  is  in  an  adjustable  sub-stage,  bring  it  as 
close  to  the  stage  as  possible,  or,  whether  here  or 
attached  to  the  stage,  it  may  as  well  be  removed  for 
the  present.  Observe  now  whether  outside  of  the  cen- 
tral rib  any  lines  can  be  seen  upon  the  surface  of  the 
diatoms  ;  if  not,  vary  the  distance  of  the  mirror  from 
the  object;  or,  if  lamp  light  is  used,  bring  the  lamp 


58 

closer  to  or  remove  it  from  the  instrument  in  one 
line,  so  that  the  illumination  will  not  disappear.  If 
this  does  not  bring  out  the  lines,  swing  the  mirror-bar 
from  the  central  position  into  an  oblique  one,  on  the 
side  opposite  to  that  of  the  light  and  readjust  the 
mirror ;  in  doing  this  grasp  the  ends  of  the  mirror- 
fork  between  the  thumb  and  middle  finger  and  move 
the  mirror  by  the  first  finger.  If  the  field  can  not  be 
evenly  illuminated,  it  is  evidence  that  the  mirror  is 
beyond  the  limit  of  angular  aperture  in  the  objective, 
and  it  must  therefore  be  brought  back  until  it  is.  It 
must  here  also  be  noticed  that  if  the  diaphragm  is 
still  attached  to  the  instrument  and  does  not  swing 
with  the  mirror,  it  may  also  be  the  means  of  cutting 
off  light.  By  means  of  the  micrometer  screw  carry 
the  fine  adjustment  back  and  forth  beyond  the  plane 
of  the  object  and  observe  closely  whether  any  lines 
can  be  distinguished.  It  is  very  probable  that  they 
will  show  ;  but  if  not,  the  cause  should  be  deter- 
mined. It  may  be  that  the  magnifying  power  is  not 
sufficiently  great,  and  in  this  case  a  higher  power  eye- 
piece should  be  used,  or  the  cover  glass  may  be  more 
or  less  than  the  normal  thickness,  which  would  cause 
a  spherical  over  or  under-correction  in  the  objective. 
In  this  case  th'e  lines  would  appear  when  the  diatom 
is  not  in  focus.  If  the  objective  is  a  non-adjustable 
one,  the  proper  correction  may  be  approximately 
reached  by  means  of  the  draw-tube.  If  the  lines 
appear  over  the  plane  of  the  object,  it  shows  over- 
correction,  and  the  length  of  tube  should  then  be 
decreased,  or  contrary  when  the  lines  show  below  or 


59 

beyond  the  plane  of  the  object.  If  the  above  direc- 
tions have  been  followed,  the  lines  cannot  fail  to  be 
seen  with  a  moderately  good  j  or  4  inch  objective  ; 
but  if  they  are  not,  the  trial  should  be  repeated. 
Again,  be  careful  to  have  no  obstruction  between  the 
course  of  rays  from  the  mirror  to  the  stage  ;  get  good 
illumination  on  the  object ;  observe  well  ;  keep  the 
instrument  in  such  a  position  that  the  object  is  not 
illuminated  from  any  other  direction  than  from  the 
mirror. 

When  the  diatoms  are  resolved  in  this  manner,  the 
lines  will  appear  to  be  diagonal  in  some  ;  longitudinal 
or  transverse  in  others,  according  to  their  position  ; 
anft,  if  the  resolution  is  very  good,  these  lines  will 
further  resolve  themselves  in  minute  beads  of  a  hexa- 
gonal form. 

It  will  now  be  well  to  bring  the  mirror  more  nearly 
to  a  central  position  ;  do  this  at  intervals  of  about  10 
degrees,  and  note  the  appearance  at  each  decrease  of 
obliquity.  It  will  be  found  that  as  the  mirrror 
approaches  the  optical  axis  the  lines  will  appear  to 
become  more  faint,  and  may  disappear  before  central 
illumination  is  reached  ;  in  this  case  it  will  be  well  to 
begin  again.  An  endeavor  should  be  made  to  make 
each  attempt  give  better  results  than  the  preceding 
one.  Repeated  trials  will  not  only  impress  the  var- 
ious phenomena  upon  the  mind,  but  will  cause  a  nota- 
ble improvement  in  manipulative  skill,  and  thus  a 
better  performance  in  the  objective. 

Until  now  we  have  assumed  that  transmitted  light 
has  been  used.  We  will  now  suppose  that  the  object 


60 

is  not  sufficiently  transparent  to  use  this  method  ;  the 
object  is  then  said  to  be  opaque,  and  requires  a  dif- 
ferent procedure. .  We  will  say  that  it  is  desired  to 
examine  an  insect  ;  it  may  be  attached  to  a  slide,  or, 
what  is  better  still,  may  be  fastened  in  a  stage  for- 
ceps (Fig.  16)  as  irmay  then  be  turned  and  viewed 
from  all  sides.  The  low  power  objective  should 
again  be  attached  ;  after  having  been  focused,  it  will 
be  found  that  the  light  is  insufficient  to  illuminatate 
n^w^-^n-^  it.  The  mirror- 
bar  should  now 
be  swung  upon 
Fig.  16.  its  axis  around 

the  stage  to  a  point  above  it,  so  it  will  be  at  an  angle 
of  about  45  degrees  to  its  surface.  If  a  lamp  is  used 
and  is  in  the  same  position  as  when  used  with  trans- 
mitted light,  it  is  probable  that  the  tube  of  the  instru- 
ment will  obstruct  the  light,  and  it  is  then  well  to 
move  it  toward  the  front.  Using  the  concave  mirror 
adjust  it  so  that  the  light  will  be  concentrated  upon 
the  object,  by  watching  it  directly,  and  then  observe 
through  the  tube.  If  it  is  not  sufficiently  illuminated 
continue  to  adjust  the  mirror  ;  also  vary  its  distance 
from  the  object  and  swing  the  mirror-bar  to  a  higher 
or  lower  point.  It  often  occurrs  that,  under  the  best 
conditions,  the  need  of  better  illumination  is  felt  ; 
in  this  case  a  bulls-eye  condenser  should  be  procured. 
It  will  be  found  that  this  will  become  a  useful  and 
perhaps  necessary  accessory  in  work  outside  of  this. 
Place  it  close  to  the  instrument  and  set  the  bulls-eye 
between  the  object  and  source  of  light,  with  \.\\e  plane 


61 


side  toward  the  object ;  if  an  ordinary  hand  lamp  is 
used,  it  will  be  necessary  to  elevate  this  to  about  the 
height  of  the  eye-piece,  and  if  it  is  to  be  used  often 
in  this  position,  a  special  support  should  be  made  for 
this  purpose. 

Low  power  objectives  are  usually  used  on  opaque 
objects,  but  sometimes  a  higher  power  is  desired. 
Unless  one  is  constructed  with  a  view  to  opaque  illumi- 
nation its  working  distance  is  usually  so  short  that  it 
will  prevent  the  light  from  striking  the  object.  A  J 
or  i  objective,  of  75  degrees,  has  sufficient  working- 
distance,  and  its  mounting  are  made  conical  in  the 
front,  so  that  it  will  allow  it. 

Dark  Ground  Illumination. — This  method  is 
not  in  general  use,  probably  because  it  requires  a 
special  accessory,  although  it  yields  beautiful  effects. 
It  is  accomplished  by  means  of  a  paraboloid  (Fig.  17) 

which  is  attached  to  the 
sub-stage.  As  will  be  no- 
ticed in  the  illustration,  the 
lower  surface  of  the  para- 
poloid  is  plane,  and  the 
light  passes  through  this 
without  undergoing  any 
change.  When  it  reaches 
the  polished  parabolic  sur- 
face it  is  reflected  to  one 
point,  according  to  the  sim- 
ple optical  law  that  the 
Ffg.  17.  angle  of  reflection  is  equal 


62 

to  the  angle  of  incidence.  An  opaque  stop,  which 
is  cemented  to  the  concave  surface,  prevents  the 
light  from  passing  through  the  central  portion  of  the 
paraboloid.  The  object  is  thus  illuminated  on  all 
sides  by  such  an  obliquity  qf  light,  that  it  does  not 
pass  into  the  objective  ;  the  object  stands  out  in 
relief,  pleasantly  illuminated  on  a  dark  back-ground. 
In  using  the  paraboloid,  the  plane  mirror  should  be 
used,  and  it  is  .necessary  to  vary  its  distance  from  the 
object  in  order  to  attain  the  best  results. 

Cover-Glass. — Thus  far  no  attention  has  been 
given  to  the  use  of  the  cover-glass,  although  it  is  an 
important  factor  in  reaching  good -results.  In  prelim- 
inary examinations  of  solid  objects  with  low  powers 
it  may  be  dispensed  with  ;  but  where  fluids  are  used, 
whether  with  low,  medium,  or  high  powers,  it  should 
always  be  used.  A  drop  or  small  quantity  of  fluid 
placed  upon  a  slide  assumes  a  spherical  form,  and,  on 
viewing  it  with  a  low  power,  it  will  be  found  to  give 
a  distorted  field,  and  will  cause  disagreeable  reflec- 
tions and  shadows. 

As  stated  before,  medium  and  high  powers  have  a 
comparatively  short  working  distance,  and  the  front 
lenses  will  be  so  close  to  the  water,  urine,  blood,  etc., 
that  the  capillary  attraction  will  often  cause  an  adher- 
ence to  the  front  surface  of'the  objective;  besides 
this,  there  is  such  a  considerable  depth  to  the  fluid 
that  it  obstructs  the  light,  requires  a  great  change  in 
adjustment  for  the  various  planes,  and  is  usually  in 
such  vibration  that  a  sharp  focus  becomes  impossible; 


63 

by  merely  dropping  a  cover-glass  upon  it  all  these 
objections  are  overcome. 

The  above  are  merely  practical  considerations,  but 
there  are  others  of  a  theoretical  nature  and  of  as 
much  importance.  After  a  high  power  objective  has 
been  corrected  to  a  certain  thickness  of  cover,  any 
variation,  not  necessary  considerable,  has  an  injurious 
effect  upon  the  spherical  corrections,  and  consequently 
upon  the  resolving  power.  It  is  manifest  that  the 
quality  of  the  latter  will  decrease  as  the  variation 
increases,  and  when  it  reaches  a  point  where  no  cover 
is  used,  it  may  be  so  considerable  as  to  destroy  an 
accurate  perception  of  what  is  sought. 

In  this  connection  it  is  considered  important  to 
state  what  thickness  of  cover-glass  it  is  best  to  use. 
As  is  probably  well  known,  there  are  three  grades, 
which  are  designated  as  No.  1,  No.  2  and  No.  3. 
Although  they  are  classified,  there  is  a  variation  with- 
in the  limits  of  different  numbers  The  variation  is 
about  as  follows  :  No  1,  T^  to  -^  inch  thick  ;  No. 
2,  yffl-  to  T^  inch  thick  ;  No.  3,  -^  to  -^  inch  thick  ; 
According  to  the  prices  of  cover-glasses,  when  pur- 
chased by  weight,  the  No.  1  give  the  greatest  number 
and  No.  3  the  least.  It  may  for  this  reason  be 
thought  that  the  purchase  of  No.  2  is  most  advan- 
tageous, but  it  must  be  considered  that  there  is  a 
greater  proportion  of  breakage  by  cleaning,  as  they 
are  very  thin  and  sensitive.  Considered  only  from  a 
optical  standpoint,  No.  2  should  generally  be  used,  as 
the  medium  and  high  power  objections  are  adjusted 


64 

to  this  thickness  and  give  the  best  results  with  the 
thinnest  of  these.  The  same  thickness  is  also  used 
on  test  objects,  but  they  are  generally  not  of  as  much 
uniformity  as  might  be  desired.  Objectives  some- 
times have  such  an  extremely  short  working-distance, 
that  it  is  necessary  to  use  the  thinnest  of  No.  1,  but 
as  these  are  usually  provided  with  adjustment  for 
correction,  their  injurious  influence  is  not  so  much 
felt.  The  thickest  covers  are  most  comfortable  to 
handle  and  may  be  used  with  low  power  objectives 
without  much  sacrifice  of  definition. 

The  writer  takes  the  liberty  of  inserting  in  this  con- 
nection extracts  from  a  paper  which  he  recently  read 
before  the  American  Society  Microscopists  and 
which  he  hopes  will  give  further  information  on  this 
subject. 

"  The  cover-glass  may  truly  be  called  a  necesssary 
evil ;  for,  while  absolutely  required  in  microscopic 
investigations,  there  is  no  adjunct  to  the  microscope 
that  has  been  and  is  productive  of  so  much  evil,  and 
has  retarded  the  utilization  of  benefits  made  possible 
by  the  advance  in  the  construction  of  objectives  so 
much  as  it. 

"  It  must  be  remembered  that  the  majority  of  objec- 
tives will  always  be  dry,  and  especially  so  when  such 
improvements,  which  we  hope  are  still  to  be  made, 
are  accomplished.  It  is  an  unfortunate  circumstance 
that  with  this  class  of  objectives  the  influence  of  var- 
iation in  thickness  of  cover-glasses  is  most  apparent ; 
but  since  it  is  so,  we  should,  if  possible,  provide  an 


65 

agency  which,  eliminating  the  personal  factor  of 
efficiency,  will  give,  under  all  conditions,  results 
closely  equal  to  those  under  which  the  objectives  were 
originally  corrected. 

"It  is  surprising  to  see  how  little  attention  is  paid  to 
this  subject  in  the  large  majority  of  standard  works 
on  the  microscope.  Almost  all  books  give  carefully 
prepared  illustrations  and  descriptions  showing  the 
effect  on  the  course  of  light  by  the  interposition  'of 
the  cover-glass,  and  after  giving  conclusive  evidence 
of  its  disturbing  influence,  still,  in  a  general  way,  say 
it  is  of  little  moment. 

"  With  such  statements  to  guide  the  microscopist, 
it  is  not  surprising  that  the  subject  should  have 
received  so  little  attention,  and  that  any  efforts  to  lead 
to  improved  methods  of  manipulating  objectives 
should  have  almost  completely  failed  because  of  a 
lack  of  the  true  understanding  of  their  need  and  con- 
sequent failure  to  create  interest.  The  belief  is  quite 
general  that  any  time  devoted  to  this  subject  is  wasted 
and  might  better  be  utilized  in  other  directions.  I 
hope  to  be  able  to  show  that  this  is  entirely  wrong, 
and  may  here  say  that,  while  I  may  be  considered  an 
extremist  in  the  other  direction,  my  efforts  emanate 
from  the  desire  to  put  it  in  the  power  of  every  micro- 
scopist to  obtain  the  highest  possible  results  from  his 
optical  battery  and  equal  to  those  obtainable  by  the 
optician. 

"Outside  of  the  differences  of  the  lengths  of  tubes 
used  by  different  makers,  which  is  also  of  great  bear- 


66 

ing  on  the  spherical  correction  of  objectives,  one  is 
astounded  by  the  difference  in  standard  cover- glasses 
used  by  different  makers  in  correcting  non-adjustable 
objectives.  With  a  thickness  of  0.10  mm.  for  the 
thinnest  and  0.25  mm.  for  the  thickest,  it  is  only  too 
apparent  that  with  the  additional  variation  in  lengths 
of  tubes  it  is  beyond  the  power  of  the  microscopist  to 
obtain  even  approximately  the  best  results  from  his 
objectives.  More  than  this,  a  large  quota  of  the 
advance  made  in  recent  years  in  the  capacity  of  objec- 
tives has  been  lost. 

"The  greatest  difficulty  is  met  with  non-adjustaleb 
objectives.  As  is  well  known,  compensation  for  thick- 
ness may  be  obtained  in  the  proper  adjustment  of 
tube  length  ;  but  while  not  all  microscopes  are  suit- 
ably provided  with  draw-tubes,  the  requisite  exper- 
ience and  skill  is  lacking  with  a  large  number  of 
microscopists  to  properly  make  the  correction  in  this 
manner,  as  well  as  in  objectives  specially  provided 
with  collar  correction.  I  am  sure  that  microscopists 
of  long  experience  will  bear  me  out  in  the  statement 
that  results  with  adjustable  objectives  depend  upon 
individual  skill,  and  that  many  such  objectives  now 
in  use  fail  to  give  results  corresponding  to  their 
capacity.  It  would  seem,  therefore,  that  any  system 
to  permit  the  full  utilization  of  the  capacity  of  objec- 
tives should  depend  on  no  personal  factor — in  fact, 
should  be  mechanical. 

"  In  an  objective  corrected  for  normal  thickness  of 
cover-glass  there  will  be  spherical  over-correction  with 


67 

thick  covers  and  under-correction  with  thin  covers, 
the  amount  of  correction  varying  in  a  different  ratio 
to  the  amount  of  variations  from  the  normal  thick- 
ness. The  chromatic  correction  will  also  lose  corres- 
pondingly, but  to  not  so  high  a  degree.  While  a  devi- 


Fig.  18. 

ation  of  a  few  hundredth  millimeters  in  either  direc- 
tion will,  perhaps,  not  signify,  that  which  occurs  in 
covers  classified  in  price-list  under  one  number  is 


68 

sufficient  to  seriously  affect  and  the  high  powers 
totally  obliterate  the  definition  which  under  normal 
conditions  it  may  possess.  The  microscopist  is  there- 
fore not  obtaining  such  results  as  his  objectives 
ought  to  enable  him  to  obtain,  and  the  efforts  of  the 
conscientious  optician  to  provide  classified  objectives 
of  reliability  and  similar  performance  is  almost 
entirely  nullified. 

"The  system  which  I  have  devised  to  aid  in  overcom- 
ing these  difficulties  depends  in  the  first  instance  up- 
on a  micrometer  for  measuring  the  thickness  of  cover 
glass.  See  Fig.  18. 

"In  objectives  provided  with  cover  correction  the 
graduation  is  so  arranged  as  to  read  to  yi-¥  mm.  No 
matter  what  the  power  of  objective,  the  number  gives 
proper  correction  for  a  thickness  corresponding  to  it. 
Thus,  with  a  cover  glass  of  0.20  mm.  the  collar  of 
such  an  objective  need  merely  to  be  set  at  20  to  give 
the  proper  correction  and,  consequently,  the  best 
results. 

"All  the  other  scales  give  the  correct  tube  length 
in  inches  and  millimeters  for  covers  corresponding 
to  them,  and  in  this  manner  offer  a  ready  and  definite 
means  of  correction.  The  tube-lengths  required  for 
the  thinnest  and  thickest  covers  are  so  extreme  that 
probably  no  convenient  means  for  obtaining  them  can 
be  practically  arranged,  but  they  can  be  so  approxi- 
mately if  not  entirely.  At  any  rate,  the  micrometer 
will  detect  the  requirements  before  using  the  covers, 


69 

and  those  deviating  considerably  from  the  normal  can 
be  used  on  objects  for  use  with  low  powers  only,  in 
which  case  the  effect  will  not  be  very  appreciable. 

"In  this  system  I  do  not  overlook  the  fact  that  var- 
iation in  tube  length  involves  a  variation  in  magnify- 
ing power  ;  but,  except  in  cases  where  micrometers 
are  used,  I  consider  this  of  secondary  importance,  as 
it  always  is  in  comparison  to  results  obtained  in  resolv- 
ing and  denning  power. 

"This  system  involves  four  conditions  : 

First. — That  all  cover  glass  be  measured  before 
using  them,  and  that  the  thickness  be  noted  on  the 
preparation. 

Second. — That  for  convenience  all  draw-tubes  be 
marked  in  inches  or  millimeters  or  both. 

Third. — That  adjustable  objectives  be  corrected 
according  to  this  scale. 

Fourth. — That  the  same  tube  length  and  cover 
glass  thickness  be  used  in  all  original  corrections  of 
objectives." 

To  Draw  Objects. — It  is  very  important  that 
the  appearance  of  an  object  should  be  put  upon  paper, 
especially  of  one  which  is  not  permanently  mounted. 
To  do  this  does  not  require  any  great  amount  of  skill 
as  the  lines  which  are  projected  upon  paper  are 
merely  followed  out ;  but  it  is  necessary  that  those 
drawings  be  made  truthful.  Nothing  should  be  put 
down  which  is  not  actually  seen  ;  neither  should  any- 


70 

thing  be  omitted.  Drawings  thus  made  form  a  valu- 
able record,  not  only  for  the  individual,  but  for  others 
who  are  following  the  same  line  of  study. 

Camera  Lucida— How  to  Use  it. — The  camera 
lucida  is  the  apparatus  by  means  of  which  drawings 
are  made.  There  are  various  forms  of  these.  The 
methods  of  attaching  to  the  tubes  are  also  numerous, 
but  a  very  simple  and  effective  device  is  that  shown 


Fig.  19- 

in  Fig.  19.     The  mounting  is  fixed  to  the  cap  of  the 
eye-piece  by  means  of  a  flexible  grooved  ring. 

In  this  form  the  pencil  point  is  not  plainly  visible, 
and  a  number  of  other  forms  are  in  use  which  are 


n 

constructed  with  a  special  view  to  avoid  this  diffi- 
culty. They  are  all,  however,  considerably  more 
expensive,  but  should  be  procured  if  means  will 
permit. 

The  procedure  of  working  should  be  about  as  fol- 
lows :  Focus  upon  the  object  and  then  incline  the 
body,  so  that  the  center  of  the  eye-lens  will  be  10 
inches  from  the  table.  To  obviate  repeated  measure- 
ments, a  standard  stick  of  this  length  may  be  used. 
If  the  instrument  is  so  low  that  it  will  not  allow  the 
inclination  of  the  body  to  an  angle  of  at  least  45 
degrees  when  at  this  distance,  it  should  be  placed 
upon  a  box  ;  or,  if  not  too  high,  upon  the  case  of  the 
microscope.  Now  readjust  the  mirror  and  attach  the 
camera  lucida  from  below  and  place  the  paper  under 
the  instrument ;  look  into  the  camera  lucida  from 
above,  being  careful  that  the  eye  is  directly  over  the 
center  of  its  opening,  and  the  image  will  be  found  to 
be  projected  upon  the  paper.  Possibly,  and  very 
probably,  it  will  appear  faint.  This  is  due  to  the  fact 
that  the  paper  is  almost  as  highly  illuminated  as  the 
field.  To  remedy  this  defect  a  cardboard  should  be 
placed  between  the  paper  and  light,  so  that  the  former 
will  be  shaded  ;  the  object  will  now  come  out  in 
strong  contrast.  Take  a  well  pointed  pencil  and  fol- 
low the  lines  in  the  image.  A  little  difficulty  may  at 
first  be  experienced  in  seeing  both  the  pencil  and 
image  at  one  time,  but  after  a  little  practice  this  is 
overcome. 


It  very  often  occurs  that  the  pencil  point  can  on 
no  condition  be  seen  distinctly,  but  this  is  usually 
due  to  abnormal  sight,  with  which  persons  are  often 
afBicted.  In  these  cases,  the  glasses  which  are 
required  for  reading  should  also  be  used  in  drawing. 
The  difficulty  is  not  experienced  in  the  image,  as  this 
can  be  adjusted  to  the  eye. 

Determining  the  Magnifying  Power. — Al- 
though a  magnifying  table  may  be  furnished,  this 
gives  the  powers  merely  approximately,  as  more  or 
less  variation  occurs  in  objectives  and  eye-pieces  of 
the  same  kind.  As  it  is  interesting,  and  sometimes 
important,  to  know  the  exact  magnifying  power,  a 
simple  method  is  mentioned.  Procure  a  stage  micro- 
meter, divided  into  Ti0  inch  and  TT5Vo  incn>  an<3  per- 
haps ^oVo"  inch,  or,  if  prefered,  any  suitable  division 
in  millimetre.  Place  the  micrometer  on  the  stage, 
focus  and  incline  the  microscope,  as  if  for  drawing, 
to  within  10  inches  from  the  table  and  attach  the 
camera  lucida  ;  for  low  powers,  TfJ1F  divisions  may  be 
used  ;  for  the  higher  ones  TIJVo-  or  more  ;  the  division 
as  now  projected  may  be  marked  upon  the  paper  and 
then  measured  off  with  a  rule  divided  into  inches  and 
J^  inches ;  if,  for  instance,  the  ^^  divisions  are 
used,  and  one  division  on  the  paper  covered  1  inch 
on  the  rule,  it  is  evident  that  the  magnifying  power 
is  1,000  times  ;  if  it  covered  -f^,  equal  to  £,  on  the 
rule,  it  would  be  200  diameters,  and  so  on. 

Measuring  the  Size  of  an  Object.— A  simple 
and  reliable  way  of  learning  the  size  of  an  object  is 


73 

by  means  of  an  eye-piece  micrometer.  As,  however, 
this  does  not  measure  the  object  directly,  but  only  its 
image,  the  first  part  of  the  process  makes  it  more 
complicated.  However,  this  portion  is  usually 
attended  to  by  the  manufacturers.  The  eye-piece  is 
provided  with  a  slot,  into  which  a  micrometer  is  fitted. 
A  micrometer  with  the  same  divisions  as  the  eye- 
piece micrometer  is  placed  upon  the  stage  and  the 
objective  focussed  upon  it.  It  is  now  observed  how 
many  of  the  divisions  of  the  eye-piece  micrometer 
are  contained  in  the  magnified  division  of  the  stage 
micrometer,  and  the  resulting  figure  is  placed  in  the 
sub-division  under  the  objective.  To  determine  the 
actual  size  of  an  object,  this  is  now  placed  on  the 
stage  and,  noting  the  number  of  divisions  which  cover 
it,  these  are  divided  by  the  number  on  the  card,  and 
the  resulting  figure  gives  the  actual  size.  Suppose 
the  figure  on  the  card  is  8.0  and  the  image  of  the 
object  covers  40  of  the  spaces  which  are  divided  into 
-jL  millimeters,  the  size  of  the  object  would  be  ^  or 
L  millimeter  ;  or  expressed  in  inches,  (25.4  mm.  equal 
to  1  inch)  about  -fa. 


ADVANCED  MANIPULATION. 


Dry  Adjustable  Objectives. — The  information 
which  has  thus  far  been  given  on  the  manipulation  of 
the  microscope  may  be  termed  initiatory,  as  it  is  sup- 
posed (at  any  rate  hoped)  to  have  disclosed  some 
new  principles.  These  are  comparatively  simple,  and 
with  a  moderate  amount  of  attention,  are  easily 
acquired.  It  is  the  intention  now  to  speak  of  some- 
thing more  complex  and  to  give  instruction  in  the 
use  of  higher  grade  adjustable  and  immersion  objec- 
tives. The  difficulty  of  doing  this  increases  with 
each  step  of  advance,  and  whether  it  can  be  over- 
come by  means  of  written  words,  is,  perhaps,  an  open 
question.  However,  the  writer  is  certain  that  if  the 
following  instructions  are  faithfully  adhered  to,  satis- 
factory results  will  be  gained.  The  highest^ attain- 
ment must  of  necessity  be  the  result  of  perseverance 
and  knowledge  of  the  various  properties  of  an  objec- 
tive, which  are  given  in  preceding  pages. 

It  is  assumed  that  a  dry  objective  is  used,  say  a  |- 
140  degrees  or  ^  135  degrees,  and  provided  with 
screw-collar  adjustment  for  cover-glass  thickness  ;  it 
is  further  assumed  that  in  the  first,  the  variation 


75 

between  the  two  first  systems  (anterior  and  middle) 
is  attained  by  means  of  a  rectilinear  motion  to  the 
middle  and  posterior  system  and  stationary  anterior 
system,  while  in  the  second  the  conditions  are 
reversed  ;  the  two  posterior  systems  remain  station- 
ary while  the  front  is  adjusted.  Both  are  arranged 
with  graduations  upon  the  screw-collar  and  have  an 
index. 

In  the  objectives  of  the  Bausch  &  Lomb  Optical 
Co.,  the  graduations  range  from  10  to  35,  and  as  pre- 
viously stated  these  numbers  indicate  the  proper 
points  of  correction  for  the  respective  thickness  of 
cover  glass. 

In  these  objectives  the  open  point,  /.  e.,  where  the 
objectives  are  most  widely  separated,  is  at  10,  thus 
giving  the  correction  for  practically  the  thinnest  cov- 
ers. As  the  objective  is  moved  toward  the  higher 
numbers  the  adjustment  is  closed  and  gives  correction 
for  the  thicker  covers.  In  objectives  of  other  manu- 
facture where  this  system  is  not  adopted,  the  figures 
are  arbitrary  and  have  no  special  significance.  While 
the  open  and  closed  point  can  be  learned  in  these  by 
trial,  it  should  be  given  by  the  maker  before  work  is 
attempted  with  it.  Before  the  objective  is  attached 
the  adjustment  should  be  closed,  as,  if  this  is  neg- 
lected and  the  objective  has  a  short  working  distance, 
the  front  lens  may  come  in  contact  with  the  cover 
when  it  is  endeavored  to  focus  on  the  object. 

Probably  the  best  object  for  studying  the  effect  of 
the  screw-collar  adjustment  and  acquiring  skill  in 
determining  its  best  point  is  again  P.  angulatum. 


76 

Place  a  slide  of  this  upon  the  stage,  and  with  a  low 
power  eye-piece  select  a  diatom  which  appears  to  be 
flat  ;  such  a  one  may  usually  be  found  when  there 
are  a  number  on  the  slide.  After  the  objective  has 
been  attached  to  the  nose-piece,  focus  carefully  and 
observe  whether  any  lines  can  be  seen  ;  if  not,  grasp 
the  milled  edge  of  the  adjustment  collar  between  the 
thumb  and  first  finger  of  the  left  hand,  keeping  the 
fingers  of  the  right  hand  upon  the  micrometer  screw, 
or  vice  versa,  if  from  the  outset  it  was  made  a  habit 
to  use  the  left  hand  on  the  fine  adjustment  ;  turn  the 
collar  slightly  toward  its  open  point,  and  as  this  will 
place  the  object  out  of  focus,  move  the  fine  adjust- 
ment correspondingly  ;  continue  to  turn  the  collar, 
little  by  little,  and  do  not  cease  to  observe  closely ; 
also,  after  each  movement,  focus  above  or  below  the 
plane  of  the  object,  so  that  this  will  be  indistinct,  and 
look  for  the  lines.  .  Possibly  after  a  little  they  will 
begin  to  appear  faintly  ;  but,  if  not,  continue  to  bring 
the  collar  toward  the  middle.  The  lines  must  now 
soon  make  their  appearance,  and,  when  they  do,  it 
will  probably  be  above  the  plane  of  the  diatom.  This 
is  an  indication  that  the  objective  is  approaching  its 
correction  for  the  cover.  Now  keep  the  lines  in  focus, 
while  the  correction  collar  is  being  gradually  turned, 
until  the  lines  and  the  outline  of  the  diatom  lie  in  one 
plane;  the  objective  is  now  said  to  be  corrected  for 
cover.  Observe  which  number  corresponds  to  the 
index,  and  note  this  upon  paper ;  again  return  the 
collar  to  its  closed  point  and  go  through  the  same 


77 

proceeding  as  carefully  as  at  first, 
point  is  again  reached,  look  for  the  number  and  see 
whether  it  agrees  with  the  first  ;  very  likely  it  does 
not,  which  is  owing  to  a  want  in  the  faculty  of  per- 
ception, due  to  a  too  slight  acquaintance  with  the 
phenomena.  These  trials  should  be  repeated  until 
the  proper  sensitiveness  of  feeling  in  making  the 
adjustments  is  acquired,  and  until  they  can  be  made 
to  correspond  with  a  certainty  to  at  least  within  two 
divisions. 

Remove  the  eye-piece  and  attach  one  of  higher 
power.  It  must  how,  however,  be  remembered  that 
if  there  is  a  considerable  difference  in  the  powers 
there  will  be  a  relative  difference  in  their  lengths,  and 
that  this  will  cause  a  difference  in  the  optical  length 
of  the  tube  ;  this  not  only  will  require  another  adjust- 
ment for  focus,  but  will  partially  destroy  the  correc- 
tion as  made  with  the  low  power.  After  some  prac- 
tice, the  amount  of  variation  may  be  fixed  upon  and 
may  be  noted  for  the  future  ;  but,  to  determine  it,  the 
same  plan  as  suggested  with  the  low  power  eye- piece 
should  be  followed. 

When  it  is  fo^nd  after  repeated  trials  that  sufficient 
skill  has  been  acquired  to  bring  the  collar  to  within 
one  division,  the  number  and  power  of  the  eye-piece 
should  be  scratched  with  a  diamond  upon  the  slide  or 
with  pen  and  ink  upon  the  label  ;  thus,  if  it  is  found 
that  with  a  1J  inch  eye-piece  the  index  shows  5,  and 
with  a  I  inch  eye-piece  shows  5£,  it  should  be  marked 
U-5  and  |-54.  In  eye-pieces  of  par-focal  construe- 


78 

tion  it  will  be  unnecessary  to  mark  with  different 
figures,  as  the  correction  will  be  the  same  for  all 
powers.  For  future  examinations  on  the  same  slide, 
this  will  facilitate  work  and  give  the  assurance  that 
the  best  results  are  thus  gained  without  further  trial. 
Mr.  Wenham's  general  rule  for  obtaining  the  best 
correction  on  objects  in  general  is  as  follows  :  "  Select 
any  dark  speck  or  opaque  portion  of  the  object  and 
bring  the  outline  in  perfect  focus  ;  then  lay  the  finger 
on  the  milled  head  of  the  fine  adjustment  and  move 
it  briskly  backwards  and  forwards  in  both  directions 
from  the  first  position.  Observe  the  expansion  of  the 
dark  outline  of  the  object,  both -when  within  and 
when  without  the  focus.  If  the  greater  expansion  or 
coma  is  when  the  object  is  without  the  focus  or  far- 
thest from  the  objective,  the  lenses  must  be  placed 
farther  assunder  (or  opened).  .If  the  greater  coma  is 
when  the  object  is  within  the  focus,  or  nearest  the 
objective,  the  lenses  must  be  brought  closer  together 
(or  closed).  When  the  objective  is  in  proper  adjust- 
ment, the  expansion  of  the  outline  is  the  same  both 
within  and  without  the  focus."  * 

Immersion-Adjustable    Objectives.— As  was 

stated  before,  immersion  contact  between  the  objec- 
tive and  cover-glass  is  made  by  either  water  or  homo- 
geneous fluid.  The  fluid  should  be  kept  in  a  small 
bottle  or  phial,  the  cork  of  which  is  pierced  to  receive 
a  small  pointed  stick  or  match,  and  this  should  pro- 
ject sufficiently  so  that  it  will  enter  the  fluid  about 


79 

i  inch.  The  fluid  will  then  always  be  free  from  dust 
and  by  withdrawing  the  cork  the  stick  will  always 
carry  a  drop  of  fluid  with  it. 

In  fixing  an  immersion  objective  to  the  stand,  the 
latter  should  first  be  put  in  an  upright  position  ;  the 
fluid  should  now  be  attached  to  the  front  lens,  but 
care  should  be  taken  not  to  put  on  too  much  ;  it 
should  be  merely  enough  to  cover  the  surface.  If 
too  much,  a  portion  of  it  should  be  removed  by  allow- 
ing it  to  adhere  to  the  finger.  The  objective  may 
then  be  attached  to  the  stand  and  brought  down  until 
the  fluid  is  in  contact  with  the  cover ;  the  stand  is 
now  inclined  and  the  objective  focused  ;  if  this 
method  is  followed  there  is  no  danger  of  flooding  the 
entire  cover  with  fluid,  which  sometimes  may  be  the 
means  of  destroying  the  object ;  neither  can  the  fluid 
run  out  from  between  the  two  surfaces. 

Extreme  cleanliness  should  be  observed  in  all  work 
connected  with  the  microscope,  and  particularly  in 
the  use  of  immersion  objectives.  The  use  of  immer- 
sion fluid  in  itself  involves  a  certain  amount  of  incon- 
venience, but  as  in  many  cases  it  is  absolutely  neces- 
sary, the  observance  of  fixed  rules  will  materially  help 
to  overcome  some  of  the  disagreeable  features.  After 
the  work  with  an  immersion  objective  has  been  com- 
pleted, the  objective  should  be  removed  from  the 
stand,  and  its  front,  as  well  as  the  slide,  should  invar- 
iably be  cleaned  ;  the  fluid  may  be  removed  by  a  moist 
piece  of  soft  linen  and  then  cleaned  with  a  dry  piece; 
chamois  skin  is  not  suitable,  as  it  does  not  absorb  the 
fluid. 


80 

Test-Plate. — Almost  all  microscopists  who  take 
an  active  interest  in  the  capacity  of  their  instruments, 
supply  themselves  with  a  set  of  test  objects,  of  which 
P.  an^iilatiiin  is  in  most  general  use,  or  with  a  so- 
called  test-plate.  These  plates  consist  either  of  a  ser- 
ies of  bands  of  finely  ruled  lines  ranging  from  5,000 
to  the  inch  to  120,000  to  the  inch  or  with  a  series 
of  diatoms,  upon  which  the  markings  represent 
certain  divisions  of  an  inch.  The  one  of  these 
which  is  principally  used  is  made  by  J.  D.  Moeller, 
and  consists  of  a  series  of  20  diatoms.  They  are 
furnished  mounted  both  dry  and  in  balsam,  but 
the  latter  is  the  most  common.  Below  is  a  table  giv- 
ing the  numbers,  names  of  the  various  diatoms  and 
divisions  on  their  surfaces  to  Ty1o^  inch  A  specimen 
of  Eupodiscus  Argus  begins  and  ends  the  series  : 

Striae  in 
ToVo  of  an  inch- 

1.  Triceratium  Favus  Ehrbg 3.1  to    4. 

2.  Pinnularia  nobilis  Ehrbg 11.7  to  14. 

3.  Navicula  Lyra  Ehrbg.  var 14.5  to  18. 

4.  Navicula  Lyra  Ehrbg 23.    to  30.5 

5.  Pinularia  interrupta  Sm.  var 25.5  to  29.5 

0.  Stauroneis  Phoenicenteron  Ehrbg...    31.    to  30. 5 

7.  Grammatophora  marina  Sm 36.    to  39. 

8.  Pleurosigma  balticum  Sm 32.    to  37. 

9.  Pleurosigma  acuminatum  (Kg.)  Grun   41.    to  46.5 

10.  Nitzschia  Amphioxys  Sm 43.     to  49. 

1 1.  Pleurosigma  angulatum,  Sm 44.    to  49. 

12.  Grammatophora   oceanica    Ehrbg  = 

G.  subtilissima. .  .    60.    to  67. 


81 

13.  Surirella  Gemma  Ehrbg 43.  to  54. 

14.  Nitzschia  sigmoidea  Sm 61.  to  64. 

15.  Pleurosigma  Fasciola  Sm.  var 55.  to  58. 

16.  Surirella  Gemma  Ehrbg 64.  to  69. 

17.  Cymatopleura  elliptica  Breb 55.  to  81. 

18.  Navicula  crassinervis  Breb=Frustu- 

lia  saxonica  Rabh 78.    to  87. 

19.  Nitzschia  curvula  Sm 83.    to  90. 

20.  Amphipleura  pellucida  Kg 92.    to  95. 

It  may  be  said,  and  perhaps  with  truth,  that  a  test- 
plate  does  not  belong  to  the  necessities  of  an  outfit, 
but  considering  that  it  is  a  guage,  on  which  the  opti- 
cian usually  bases  the  quality  of  his  objectives,  it  is 
valuable  to  the  owner  of  an  objective  to  be  able  to 
determine  whether,  under  his  manipulation,  the  objec- 
tive will  perform  as  well  as  is  claimed  for  it ;  due  con- 
sideration must,  however,  be  given  to  the  fact  that 
there  is  a  certain  amount  of  variation  among  differ- 
ent plates,  as  is  shown  in  the  above  table.  Outside 
of  this,  it  is  a  continual  incentive  to  determine  the 
extreme  performance  of  an  objective,  and  it  thus 
becomes  the  means  of  acquiring  great  manipulative 
skill,  which  cannot  be  underrated.  The  writer  is  in 
a  position  to  know  that  there  is  great  need  of  this ; 
innumerable  cases  have  come  to  his  notice  where  sev- 
eral objectives  of  the  same  kind  and  equal  quality 
gave  unequal  results  in  different  hands,  and  would  be 
highly  eulogized  by  the  possessor  of  one  and  con- 
demned by  that  of  another. 


82 

Immersion  Objectives  on  Test  Plate. — To 

determine  the  highest  capacity  on  test  objects,  ordi- 
nary daylight  is  hardly  sufficient ;  moderate  sunlight 
or  good  lamp-light  is  best  suited,  but  the  latter, 
from  the  fact  that  it  is  always  at  hand,  is  preferable. 
For  the  purpose  of  explanation,  we  will  assume  that  a 
flat  wick  lamp  and  a  J,  TV  or  Jg  homogeneous  immer- 
sion objective  is  used.  If  the  right  hand  is  used  on 
the  micrometer  screw,  place  the  lamp  at  the  right  side 
of  the  instrument,  about  10  inches  from  it,  with  the 
edge  of  the  flame  turned  toward  the  mirror. 

The  test  plate  may  now  be  placed  upon  the  stage, 
and  as  the  diatoms  in  balsam  are  very  transparent, 
and  therefore  very  difficult  to  find,  a  lower  power 
objective  may  be  used  as  a  finder;  bring  No.  1,  or 
Tricerdtium  Favus,  in  the  center  of  the  field,  and  after 
the  objective  has  been  removed,  attach  the  immersion 
objective  in  the  manner  prescribed  ;  the  adjustment 
collar  may  be  placed  at  zero,  as  this  is  about  the  cor- 
rect point  for  standard  length  of  tube.  Get  the  best 
possible  illumination  with  the  mirror  at  the  central 
point  and  move  the  test  plate  from  diatom  to  diatom 
until  it  reaches  No.  11,  P.  angulatum,  but  observe 
closely  the  structure  of  each  one  as  it  comes  into  the 
field.  Next  see  whether  the  objective  is  corrected  ; 
if  the  lines  and  outlines,  or  middle  rib,  do  not  appear 
to  be  in  one  plane,  adjust  the  collar  until  they  are, 
and  then  continue  the  advance  toward  the  higher  num- 
bers until  one  is  reached  on  which  no  lines  can  be 


83 

seen.  Swing  the  mirror-bar  to  an  obliquity  of  20 
degrees,  and,  readjusting  the  mirror,  observe  the 
effect.  It  is  very  probable  that  the  lines  will  show, 
and,  if  so,  continue  the  advance  ;  if  they  do  not,  give  1 0 
degrees  or  20  degrees  more  obliquity,  and  after  the 
structure  comes  out,  again  go  forward.  A  point  may 
thus  be  reached,,  where  with  the  greatest  obliquity 
which  can  be  given  and  with  the  best  possible  illumi- 
nation, the  objective  seems  to  have  come  to  the  limit 
of  its  performance.  From  the  claims  which  have  been 
made  for  it,  it  ought  to  do  better.  What  is  the  cause 
of  failure  ?  Possibly  the  mirror  is  not  correctly 
focuse'd,  or  the  adjustment  collar  may  not  be  correct 
for  oblique  light ;  perhaps  the  eye-piece  does  not  give 
sufficient  magnifying  power  to  distinguish  the  striae. 
It  may  be  any  one  of  these  causes  or  all  combined. 
As  to  the  eye  piece,  the  manipulator  must  remember 
the  amount  of  separation  of  lines  in  the  last  object 
which  was  resolved,  and  from  the  gradation  in  the 
coarser  specimens  must  judge  whether  the  power  is 
sufficient ;  it  should  be  added  that  for  any  over  No. 
14  and  under  No.  18  a  |  inch  eye-piece  should  be 
used,  and  for  those  above  No.  1 8  a  power  of  \  inch 
will  probably  be  necessary,  provided  a  -J-  or  ^  objec- 
tive is  used.  After  this  condition  has  been  complied 
with,  look  to  the  correction  collar  of  the  objective ; 
to  obtain  the  highest  results  it  very  often  occurs  that 
a  different  adjustment  is  required  for  oblique  light 
from  that  for  central  light.  Note  the  number  at 
which  it  stands,  and  then  work  it  back  and  forth, 


84 

watching  carefully  for  results.  If  this  has  no  influ- 
ence, return  it  to  its  number  or  to  a  point  where  the 
outline  of  the  object  appears  most  sharp.  Now  look 
to  the  illumination  ;  vary  the  distance  of  the  mirror 
to  the  object,  and,  if  it  conflicts  with  the  stage  or 
does  not  give  the  desired  results,  vary  the  distance  of 
the  lamp  to  the  instrument  and  watch  the  effect  of 
the  change  through  the  tube.  A  great  change  in  the 
illuminating  power  can  thus  be  produced  ;  the  light 
is  best  when  it  covers  the  least  space,  as  it  is  then 
most  intense.  The  light  may  be  quickly  adjusted  by 
throwing  it  upon  a  point  on  the  slide  in  the  opening 
of  the  stage  and  watching  it  there:  If  neither  of  these 
changes  give  any  improvement,  recourse  must  be  had 
to  another  expedient.  Place  a  bull's-eye  between  the 
lamp  and  mirror  with  the  plane  side  to  the  former, 
and  close  to  it  so  that  the  light  is  thrown  on  the  lat- 
ter. It.  should  be  properly  concentrated  so  that  the 
circle  of  light  will  not  be  larger  than  the  mirror, 
which  can  be  determined  by  placing  the  hand  or  a 
piece  of  paper  back  of  it.  Adjust  when  necessary  by 
moving  the  lamp  or  bull's  eye.  Keep  it  a  little  below 
the  line  of  the  face  of  the  stage,  so  that  the  light  will 
not  strike  it  on  its  upper  and  as  little  as  possible  on 
its  lower  surface  ;  if  the  light  from  the  bull's-eye 
directly  reaches  the  object,  it  destroys  the  effect  of 
the  oblique  illumination.  Great  care  should  be  given 
to  this  point,  as  it  is  very  important. 

If  all  of  these  suggestions  have  been  followed,  a 
great  difference  will  undoubtedly  be  noticed  in  the 


85 

performance  of  the  objective  ;  but  if  it  still  does  not 
come  up  to  the  standard,  patience  must  not  be  lost. 
The  slightest  change  in  the  mirror,  bull's-eye,  or 
lamp,  a  touch  to  the  correction  collar  or  micrometer 
screw  is  sometimes  followed  by  astonishing  results. 
The  beginner  should  sit  down  with  the  expectation 
that  he  will  fail  at  the  first  trial.  At  each  succeeding 
trial  he  can  easily  notice  his  improvement  in  manipu- 
lation and  the  gain  of  corresponding  results.  He 
should  be  able  to  bring  the  performance  of  the  objec- 
tive up  to  the  claims  made  for  it,  if  it  has  come  from 
the  hands  of  a  reliable  optician,  and  should  not  rest 
until  this  is  accomplished. 

The  writer  has  often  recommended  sunlight  with 
generally  successful  results  where  ordinary  means  of 
illumination  have  failed.  The  light  is  of  course 
intense,  and  great  care  will  have  to  be  used  to  modify 
it  by  properly  using  the  mirror,  but  success  is  often 
attained  and  then  creates  confidence.  It  is,  however, 
only  recommended  for  this  purpose  and  not  for  gen- 
eral use. 

To  the  histologist  it  may  seem  strange  that  the 
writer  has  thus  far  only  spoken  of  working  with 
objectives  on  diatoms.  This,  however,  was  done 
advisedly.  They  are  thin,  and  therefore  as  suitable 
as  a  thin  section  and  far  more  preferable  than  a  thick 
one.  Their  form  and  structure  are  easily  recogniz- 
able, and  there  is  very  little  variation  among  those  of 
the  same  kind  ;  therefore,  rules  laid  down  regarding 
them  are  generally  good.  It  is  conceded  by  advanced 


workers  that  the  time  spent  over  diatoms  for  the  pur- 
pose of  studying  objectives  is  well  applied,  and  the 
most  expert  manipulators  have  acquired  their  exper- 
ience in  this  manner.  An  objective  which  works  well 
on  diatoms  works  equally  well  on  other  objects,  and 
therefore  the  manipulative  skill  which  has  been 
attained  on  the  former  is  as  well  applied  on  the  lat- 
ter. At  the  outset  work  may  be  done  on  other 
objects  than  diatoms,  and  where  ordinary  working 
objectives,  such  as  a  Student  1  inch  and  J  inch,  or  f- 
and  |  inch  comprise  the  outfit,  the  road  to  good  manip- 
ulation may  be  as  short  as  with  diatoms  The  condi- 
tions in  both  cases  remain  the  same  ;  but  it  must  be 
cautioned  that,  if  histological  preparations  be  used, 
only  such  be  selected  as  are  reliable.  A  poor  speci- 
men is  perhaps  as  bad  as  none  at  all  ;  an  abnormally 
thick  one  obstructs  light,  makes  it  impossible  for  the 
objective  to  penetrate  through  the  various  layers,  and 
leaves  the  impression  that  the  latter  is  defective 

Photo-Micrography.  —  This  subject  does  not 
properly  belong  within  the  scope  of  this  book,  but 
there  are  some  points  connected  with  it  which  may 
be  of  value  to  mention. 

Any  person  desiring  to  do  this  work  should 
endeavor  to  obtain  some  experience  in  ordinary  pho- 
tography and  proper  developing.  First,  as  good 
results  in  photography  with  the  microscope  are  diffi- 
cult to  obtain,  even  with  previous  experience,  a  book 
devoted  to  this  purpose  should  be  well  studied  before 
attempting  to  obtain  results.  The  beginner  in  this 


87 

direction  will  find  it  somewhat  difficult  to  make  a 
proper  selection  of  apparatus.  The  most  inexpensive 
is  to  use  the  ordinary  microscope  in  a  horizontal  posi- 
tion and  attach  to  an  ordinary  camera,  in  which  care 
will  have  to  be  observed  to  see  that  the  ground  glass 
is  at  exactly  right  angles  to  the  optical  axis.  As  a 
rule  the  ordinary  view  camera  is  not  of  sufficient 
accuracy  to  use  without  specially  adapting  it  to  this 
purpose,  and  the  main  difficulty  lies  in  the  lack  of 
coincidence  of  the  ground  glass  with  the  film  side  of 
the  plate.  Plates  in  themselves  are  irregular  and  the 
shoulders  on  which  they  rest  should  at  any  rate  coin- 
cide. The  best  plan  to  determine  this  is  by  placing 
a  straight  edge  on  the  frame  of  the  ground  glass, 
interpose  a  wooden  wedge  between  it  and  the  ground 
surface  of  the  glass  and  mark  the  point  of  contact  by 
a  pencil  mark.  Follow  the  same  procedure  with  the 
plate  holder  containing  the  plate.  The  variation  can 
in  this  manner  be  seen  with  a  nicety  and  proper  cor- 
rection easily  made.  The  writer  recommends  that 
this  test  be  made  with  each  plate,  so  that  this  is 
always  in  exact  coincidence. 

Where  means  will  permit,  the  best  plan  is  no  doubt 
to  obtain  an  apparatus  which  is  complete  in  itself  and 
is  used  for  this  purpose  only.  The  Atwood,  as  an  inex- 
pensive, and  the  Rafter  as  complete  in  every  direc- 
tion, may  be  recommended. 

Whether  an  eye-piece  should  be  used  or  not  is  a 
matter  of  controversy.  Both  methods  are  followed 
with  good  success.  By  the  use  of  the  eye-piece  the 


88 

microscope  is  brought  to  its  normal  condition  and  the 
projection  of  the  image  into  the  camera  carries  with 
it  the  faults  of  the  eye-pieces,  and  these  increase  with 
the  length  of  the  camera.  Without  the  use  of  the  eye- 
piece the  spherical  and  chromatic  correction  of  the 
objectives  is  disturbed  in  proportion  as  the  plate  is 
distant  from  the  standard  tube  length  and  will  destroy 
the  definition  of  a  high  power  objective  unless  com- 
pensation can  be  made  by  means  of  collar  correction. 

The  plan  followed  by  a  well-known  worker  is  to 
take  a  negative  at  the  end  of  the  tube  without  the 
eye-piece,  where  the  proper  corrections  are  obtained, 
and  from  this  enlarge  to  any  suitable  size. 

The  amplifier  should  be  used  where  the  eye-piece 
is  not  employed,  as  by  its  proper  adjustment  the 
objective  may  be  brought  to  its  normal  condition. 
This  is  the  principle  involved  in  the  Rafter  camera. 
Its  use  is  therefore  not  that  of  an  amplifier  but  that 
of  a  corrector. 

The  question  of  suitable  objectives  is  one  of  con- 
considerable  importance.  The  ordinary  microscope 
objective  is  not  constructed  with  a  view  to  photo- 
graphic work,  and  unless  nicely  corrected  there  will 
be  a  lack  of  coincidence  between  the  visual  and  chem- 
ical rays,  /.  e.  between  the  image  which  is  seen  on  the 
ground  glass  and  that  which  is  photographically  pro- 
duced. For  all  this,  practical  experience  has  shown 
that  a  number  of  Bausch  &  Lomb  Optical  Co.  objec- 
tives are  well  suited  to  this  work.  For  low  powers 
the  Student  and  Professional  series  may  be  success- 


89 

fully  used.  For  medium  powers  special  objectives 
are  constructed,  and  for  high  powers  any  of  the  hom- 
ogeneous immersion  may  be  successfully  employed. 

Some  favor  the  apochromatic  objectives  as  these 
are  specially  constructed  with  a  view  to  using  them 
in  this  work,  but  their  considerable  higher  prices  ple- 
clude  their  use  in  many  cases. 


TO  SELECT  A  MICROSCOPE. 


When  a  person  has  concluded  to  obtain  a  micro- 
scope, a  suitable  selection  is  a  matter  of  considerable 
importance  to  him.  The  varieties  are  innumerable, 
prices  without  end,  all  sorts  of  claims  made  for  them. 
It  is,  therefore,  easily  explained -why  this  chapter 
should  be  an  important  one  in  a  manual  of  this  kind, 
and  yet  difficult  to  treat  satisfactorily. 

The  variety  of  special  lines  of  investigation  involves 
nearly  as  great  a  variety  of  requirements.  The 
amount  of  money  to  be  expended  ;  what  shall  be  the 
stand  ;  what  the  objectives  ;  shall  the  entire  outfit  be 
purchased  at  one  time  or  little  by  little,  are  all  ques- 
tions of  paramount  importance  which  the  writer  does 
not  expect  to  solve,  but  hopes  to  give  sufficient  infor- 
mation that  a  more  intelligent  selection  may  be  made 
than  might  probably  be  done  otherwise. 

Stands. — Starting  out  with  the  assumption  that 
there  are  two  classes  of  instruments  to  select  from, 
the  long  and  short  tube,  the  first  decision  to  reach 
will  be  this  point.  In  a  general  way  it  may  be  said 
that  there  are  no  optical  advantages  in  either,  but 
whichever  is  adopted  must  be  retained. 


91 

The  principal  consideration  is  whether  the  instru- 
ment is  to  be  used  in  an  upright  or  inclined  position. 
If  the  former,  the  short  tube  is  most  usually  selected, 
as  it  can  be  used  comfortably  on  a  table  of  ordinary 
height.  This  one  objection  which  might  be  and  often 
is  raised  against  the  long  tube  is  easily  overcome  by 
the  friends  of  the  latter  providing  a  suitable  table  for 
the  same.  As  the  instrument  is  used  in  the  upright 
position  only  in  a  few  special  lines  of  study,  it  is 
really  only  of  weight  in  this  direction,  as  the  instru- 
ment may  be  inclined  to  the  most  comfortable  point, 
and  when  so  is  more  comfortable  than  the  upright 
position. 

The  joint  for  inclination  of  arm  is  generally  con- 
ceded to  be  an  advantage.  While  it  may  be  the  case 
that  many  of  the  upright  instruments  are  in  use  in 
Europe,  there  are  very  few  used  in  this  country,  and 
the  preponderence  of  instruments  shown  in  catalogues 
of  foreign  makers,  would  indicate  the  same  tendency. 

Almost  all  instruments  for  reliable  work  are  pro- 
vided with  both  fine  and  coarse  adjustment.  They 
are  both  necessary,  the  only  question  being  whether 
the  latter  shall  be  by  the  sliding  tube  or  rack  and 
pinion.  The  former,  while  perhaps  having  the  advan- 
tage of  admitting  a  more  speedy  change  of  objectives, 
has  a  decided  disadvantage  in  the  hands  of  the  stu- 
dents in  endangering  objectives  and  preparations. 
Further  than  this,  it  is  almost  impossible  for  the 
maker  to  center  the  nose-piece  with  the  tube,  so  that 
a  change  of  objectives  usually  loses  an  object  out  of 


92 

the  field,  and  requires  that  it  be  looked  for  anew  with 
each  change.  In  the  rack  and  pinion  the  nose-piece 
has  an  unvarying  relation  to  the  tube,  and  is  not 
liable  to  this  difficulty,  and  offers  a  steady  and  agree- 
able adjustment.  The  advantages  of  the  rack  and 
pinion  seem  to  be  generally  appreciated  in  this  coun- 
try, for  there  are  few  instruments  sold  and  used  with- 
out it. 

Whether  an  instrument  shall  be  of  japanned  iron  or 
lacquered  brass  is  probably  largely  determined  by  the 
amount  of  money  to  be  expended.  As  far  as  the 
intrinsic  suitability  of  the  metals  is  concerned,  there 
is  no  difference.  Brass,  however,  oilers  the  maker  a 
better  opportunity  for  displaying  his  mechanical 
skill,  and  while  it  is  no  doubt  true  that  many  highly 
finished  instruments  are  of  poor  workmanship  in  their 
working  parts,  it  is  also  a  fact  that  a  well  made  instru- 
ment is  always  nicely  finished. 

The  size  of  instrument  is  worthy  of  consideration. 
If  an  instrument  is  to  remain  stationary  in  a  practi- 
tioner's office  or  laboratory,  it  may  be  large  without 
being  cumbersome.  If,  however,  it  is  intended  to  be 
carried  about,  it  should  be  of  the  smaller  and  more 
contracted  style. 

Another  important  consideration  is  the  space 
between  the  stage  and  base,  or  table.  While  it  is 
advisible  to  have  the  stage  low  on  account  of  the  con- 
venience in  manipulating  a  slide,  there  should  still  be 
sufficient  space  for  the  convenient  attachment  of  sub- 
stage  accessories.  As  a  rule  the  American  pattern  of 


93 

instruments  provide  more  room  between  the  stage  and 
base  on  the  lower  side  and  stage  ana  ^trrwfon  the 
upper,  than  do  the  Continental. 

As  stated  previously,  a  variety  of  stages  are  offered 
on  instruments  of  similar  construction.  The  plain 
flat  stage  while  preferred  to  some,  offers  no  advan- 
tages over  the  ordinary  round  one,  unless  specially 
made  for  examining  specimens  on  larger  slides  than 
the  standard  3  by  1  inch.  Some  claim  advantages  for  a 
smaller  stage  than  the  length  of  the  slide,  so  that  this, 
projecting,  admits  of  the  slide  being  grasped  and 
swung  around  the  optical  axis.  These  advantages, 
however,  are  not  generally  appreciated,  and  even  if 
so,  are  offset  by  the  drawback  that  in  moving  the 
slide  it  is  apt  to  be  tilted. 

Spring  clips  are  usually  of  similar  construction, 
although  varying  in  detail  and  curves.  Properly  con- 
structed clips  should  have  such  thickness  of  metal 
and  be  so  bent  as  to  allow  specimens  to  be  brought 
under  them  without  resistence  and  keep  them  prop- 
erly in  place  without  too  much  pressure  and  conse- 
quent friction. 

A  glass-stage  and  slide-carrier  may  be  considered 
a  good  investment,  as  it  admits  of  the  convenient 
manipulation  of  the  slide  without  the  grating  feeling 
which  usually  accompanies  the  direct  movement  of 
the  slide  on  the  stage. 

Where  systematic  examinations  of  a  specimen  are 
to  be  made,  a  mechanical  stage  will  be  found  a  great 
convenience  and  in  petrographical  work  is  almost  a 
necessity. 


94 

A  sub-stage  may,  in  a  general  way,  be  said  to  be 
preferable  when  it  is  adjustable,  particularly  in  the 
use  of  a  condenser.  It  is  absolutely  necessary  to 
adjust  the  condenser  for  different  objectives  and  this 
must  be  done  so  nicely  that  a  sliding  movement  is 
hardly  sufficient.  In  biological  work  the  condenser 
is  at  the  present  day  a  necessity,  and  this  is  now  con- 
structed as  a  separate  attachment  with  all  the  neces- 
sary adjustments,  in  which  the  rack  and  pinion  is 
deemed  of  first  importance. 

Objectives  and  Eye-Pieces. — It  is  hoped 
that  the  information  given  of  the  various  qualities  in 
an  objective  will  aid  to  make  a  suitable  selection  of 
the  optical  parts.  As  the  stands  have  been  classified 
in  long  and  short  standard  tubes,  the  first  quality  to 
look  for  is,  after  the  stand  has  been  selected,  their 
suitability  to  it. 

As  will  have  been  seen  under  the  proper  head,  a 
variety  of  powers  is  obtained  by  a  suitable  combina- 
tion of  eye-pieces  and  objectives,  and  while  power 
alone  can  be  obtained  by  increasing  the  power  of  the 
eye-piece,  it  is  not  advantageous  to  do  so.  For  ordi- 
nary work  no  higher  eye-piece  than  a  |  should  be 
used.  In  catalogues  many  outfits  are  made  up  of  one 
eye-piece  and  two  objectives,  but  this  is  only  for  the 
purpose  of  reducing  price  to  a  minimum.  It  is  always 
advisable,  when  means  will  permit,  to  select  two  eye- 
pieces, preferably  the  2  inch  and  1  inch,  and  insist 
they  be  par-focal,  as  this  will  be  found  extremely 
convenient  and  will  not  disturb  the  optical  standard 


95 

length.  If  for  any  work  J  inch  or  higher  powers  are 
desired,  the  solid  eye-pieces  may  be  recommended. 
The  periscopic  are  advantageous  for  micrometric 
tests  and  other  work  where  a  large  or  flat  field  is 
desirable. 

For  student's  and  practitioner's  use,  the  outfits  as 
made  up  in  catalogues  are  usually  sufficient,  except, 
as  above  recommended,  where  but  one  eye-piece  is 
given,  it  is  well  to  select  two  where  means  will  permit. 

Referring  particularly  to  the  catalogue  of  the 
Bausch^c  Lomb  Optical  Co.,  we  will  state  the  purpose 
for  which  they  were  intended 

Biological—  Laboratory,  student  and  professional 
use. 

Large  Biological—  Advanced  laboratory,  student  and 
professional  use. 

Harvard — Laboratory,  student  and  professional  use. 

Model — Laboratory,  student,  professional  and  ama- 
teur use. 

Physician — Professional. 

Investigator — Student,  professional  and  amateur  use. 

Universal — Student,  professional  and  amateur  use. 

Concentric — Professional  and  amateur  use. 

Professional  —  Advanced  laboratory,  professional 
and  amateur  use. 

For  ordinary  professional  use,  including  urinary 
examinations,  the  J  inch  27  degrees  and  \  inch  110 
degrees  objectives  will  be  found  sufficient.  For 
bacteriological  examinations  a  higher  power,  such  as 
a  ^  inch  homogeneous  immersion,  objective  will  be 
necessary. 


96 

If  means  will  permit,  an  investment  in  the  f  inch 
40  degrees  and  -|-  inch  130  degrees,  or  f-  inch  40  de- 
gress and  i  inch  140  degrees,  will  be  well  applied. 
This  latter  is  in  very  general  use  and  may  be  highly 
recommended. 

In  botanical  work  a  lower  power  than  those  men- 
tioned, such  as  a  -2  inch  12  degrees  or,  preferably,  2 
inch  15  degrees,  will  be  necessary. 

For  amateur-  use  the  ordinary  outfit  of  }  inch  27 
degrees  and  -J-  inch  110  degrees  with  the  addition  of 
a  2  inch,  preferably  of  the  better  grades,  will  do.  If 
the  examination  of  diatoms  will  be  followecf,  the  -J- 
inch  140  degrees  and  T12  inch  homogeneous  immer- 
sion, will  probably  be  required. 

For  the  student,  the  f  inch  28  degrees  and  £  inch 
110  degrees,  or  f  inch  27  degrees  and  -J-  inch  110 
degrees  objectives,  will  ordinarily  be  ample. 

Although  from  an  optical  standpoint  it  is  true  that 
objectives  give  more  detail  as  they  increase  in  their 
angular  apertures,  it  will  have  been  seen  that  the 
highest  class  of  objectives  is  not  always  recommended. 
A  great  portion  of  everyday  work  does  not  require 
this  maximum  of  optical  results,  and  can  be  accom- 
plished completely  and  with  comfort  with  objectives 
of  comparatively  low  aperture.  Some  years  ago  micro- 
scopists  were  divided  into  two  classes,  the  new  school 
of  wide  aperture,  and  the  old  school  of  narrow  aper- 
ture. The  state  of  affairs  existing  then  has  happily 
changed,  concessions  having  been  gradually  made,  so 
that  now  the  advantages  of  both  classes  of  objectives 


97 

are  appreciated  and  there  are  few  microscopists  of 
standing  who  would  recommend  only  one  or  the  other 
kind. 

In  these  days  of  competition,  prices  alone  are  too 
often  made  the  object  of  inducement,  without  any 
reference  to  quality.  Be  distrustful  of  all  such  objec- 
tives, arid  if  contemplating  their  purchase,  always 
reserve  the  right  of  having  them  examined  by  an 
expert.  Have  a  distrust  especially  of  all  "nameless" 
objectives.  It  is  safe  to  assume  that  if  the  maker  can 
not  attach  his  name  he  is  dpubtful  of  their  superior- 
ity. Any  maker  of  responsibility  will  say  without  hes- 
itation that  he  can  produce  objectives  at  less  than 
one -half  their  present  cost,  if  he  had  the  assurance 
that  they  would  be  accepted  as  first  put  together,  as 
the  cost  of  merely  making  and  mounting  lenses  is 
considerably  less  than  the  cost  of  making  proper  cor- 
rections. In  this  case,  however,  they  would  be  of 
varying  and  inferior  quality. 

It  is  sometimes  found  that  dealers  offer  the  same 
objectives  of  different  quality  at  different  prices.  Too 
great  care  cannot  be  observed  in  such  cases,  as  the 
very  fact  of  the  admission  of  a  difference  in  quality 
indicates  that  they  are  made  by  an  unreliable  maker. 
This  mode  of  offering  objectives  was  in  vogue  many 
years  ago  when  the  principles  of  optics  and  facilities 
for  making  were  limited,  and  when  a  higher  price  was 
asked  for  those  which  might  be  termed  a  happy  com- 
bination. There  is  no  excuse,  however,  at  the  pres- 
ent day,  for  anything  of  this  kind,  because  every  con- 


98 

scientious  optician  has  his  standard  for  every  objec- 
tive which  is  his  guide. 

In  purchasing  a  microscope  a  beginner  may  be  eas- 
ily misled  by  the  enticing  appearance  of  an  object, 
which  may  be  due  not  so  much  to  the  instrument  as 
to  the  object  itself,  and  if  the  optical  parts  are  infer- 
ior, it  will  require  but  a  short  experience  to  become 
convinced  of  it — usually  as  soon  as  a  comparison  can 
be  made  with  reliable  work.  The  investment  in  one 
of  these  objectives  is  not  only  a  source  of  disappoint- 
ment, but  usually  proves  to  be  a  pecuniary  loss,  as  it 
is  usually  followed  by  a  fresh  outlay  in  responsible 
work. 

It  is  of  ordinary  occurrence  that  such  objectives  as 
have  just  been  spoken  of  are  sent  to  the  writer's  firm 
with  the  request  to  examine  them  and  rectify  the 
faults  ;  but  an  examination  almost  invariably  proves 
that  the  cost  of  doing  this  is  considerably  greater 
than  purchasing  a  new  objective  of  the  same  power, 
and  it  would  not  even  then  be  equal  to  the  latter. 

Accessories. — As  has  been  stated  before,  one  of 
the  most  useful  accessories  to  the  stand  is  the  glass- 
stage  and  slide-carrier. 

Another  accessory  which  is  in  equal  demand,  and 
deservedly  so,  is  the  double  nose-piece.  By  means  of 
it  two  objectives  may  be  kept  permanently  attached 
to  the  microscope,  avoiding  any  loss  of  time  from 
changes.  When  properly  fitted,  an  immediate  change 
can  be  made  with  the  additional  advantage  of  having 


99 


the   objectives  centered   and  adjusted  for  focus,  or 
nearly  so,  which  is  a  very  decided  consideration. 


Fig.  20. 

The  same  advantage  holds  good  for  the  triple  nose- 
piece  in  the  case  of  three  objectives,  or  the  quadruple 
nose-piece  for  four.  In  these  it  is  usually  not  con- 
venient to  arrange  the  objectives  to  correspond  in 
focus  on  account  of  the  considerable  difference  in 
focal  distance. 

The  next  in  order  of  demand  is  the  Abbe  conden- 
ser in  its  various  forms  of  mounting.  While  some 
able  microscopists  hold  that  the  proper  use  of  the 
mirror  will  satisfy  every  demand,  and  that  unless 
properly  used  the  condenser  is  disadvantageous 
instead  of  beneficial,  it  is  unquestionably  true  that  it 
will  give  results  which  the  mirror  alone  can  not  pro- 
perly give.  Furthermore  it  enables  results  to  be 
obtained  easily,  which  would  require  careful  and  long 
experience  by  the  mirror.  The  writer  is  in  the  best 
position  to  know  from  the  many  inquiries  received 
from  professional  men,  on  how  to  increase  the  light, 
that  for  this  purpose  and  the  better  distinguishment 


100 

of  structure  alone,  it  may  be  considered  a  boon.  Bio- 
logical work  cannot  be  successfully  prosecuted  with- 
out it. 

The  polariscope  is  absolutety  requisite  in  petro- 
graphical  work,  and  in  the  examination  of  crystals  in 
connection  with  a  selenite,  it  is  an  excellent  appar- 


Figs.  21. 

atus  in  the  hands  of  professional  and  amateur  for 
instruction  and  amusement,  bringing  out  wonderful 
coloration  of  many  objects.  Although  the  microscope 
is  really  an  instrument  for  scientific  research,  it  does 
not  suffer  by  being  used  as  a  means  for  recreation 
and  pleasure,  and  in  this  direction  no  accessory  will 
aid  more  than  a  polariscope.  Three  selenites  may  be 
used  with  it,  giving  the  following  combination  of  col- 
ors :  Blue  and  yellow,  red  and  green,  purple  and 
green.  The  polarizer  is  always  arranged  to  resolve. 
The  bull's-eye  condenser  is  mainly  intended  for  the 
illumination  of  opaque  objects,  and  although  modern 
instruments  have  their  mirrors  so  hung  that  they  may 


101 

be  brought  above  the  stage  for  this  same  purpose, 
the  results  are  not  so  satisfactory.  The  bull's-eye  may 
be  used  by  interposing  it  between  the  mirror  and 
source  of  light,  and  with  a  very  material  increase  in 
its  volume  and  easy  regulation. 


Fig.  22. 

The  Mechanical  Stage,  Micrometer,  Camera  Lucida 
and  other  accessories  have  been  treated  separately, 
and  will  therefore  require  no  farther  enumeration. 


SUB-STAGE  ILLUMINATION. 


While  the  objective  and  its  proper  application  and 
use  will  always  be  the  most  important  part  of  the 
microscope,  the  proper  illumination  of  objects  is 
second  in  importance. 

Mirror. — As  has  been  stated,  the  mirror  alone 
offers  an  excellent  means  of  properly  displaying  an 
object  when  intelligently  used,  for  most  of  the  low 
and  medium  power  objectives.  On  account  of  the 
limited  capacity  of  old  objectives,  efforts  have  been 
made  for  the  past  fifty  years  to  increase  their  utility 
by  the  aid  of  lenses  below  the  object.  This  was 
mainly  in  the  way  of  increase  of  intensity  which  at 
the  present  day  is  not  of  so  much  consequence,  as 
our  sources  of  illumination  and  light  conveying 
capacity  of  objectives  is  so  much  greater. 

Abbe  Condenser. — The  history  of  sub-stage  con- 
densers is  very  unique  and  interesting,  and  shows 
how  from  having  been  the  subject  of  no  end  of  con- 
demnation it  is  now  the  reverse.  From  single  lenses, 
compound,  non-achromatic  and  achromatic,  the  use 
of  eye-pieces  and  objectives  with  any  number  of 


103 

devices  for  regulating  the  light,  the  generally  accepted 
forms  are  those  devised  by  Prof.  Abbe.  One  of  them 
with  a  numerical  aperture  of  1.20  consists  of  a  com- 
bination of  two  lenses,  and  the  other  with  an  aper- 
ture of.  1.42,  of  three  lenses.  These  condensers  are 
applied  to  microscopes  by  each  maker  in  a  different 
way,  each  seeking  in  his  own  manner  to  provide 
means  for  controlling  the  volume  or  angle  of  light  as 
well  as  to  obtain  any  degree  of  obliquity.  To  bring 
the  condenser  to  the  minimum  of  cost,  the  Bausch  & 
Lo'mb  Optical  Co.  quote  both  condensers  in  plain 
adapters,  giving  the  entire  condensing  capacity  which 
may  be  varied  only  by  varying  their  distance  from 
the  object.  In  this  form  they  are  incomplete,  how- 
ever, as  their  effectiveness  depends  upon  the  nicety 
with  which  the  light  can  be  controlled.  The  most 


Fig.  23. 

simple  device  for  this  purpose  consists  of  the  mount- 
ing with  a  slide  in  which  various  diaphragms  wilh  cen- 
tral stops  and  openings  may  be  dropped  into  it,  and 


104 

brought  into  the  optical  axis  or  passed  diametrically 
across  the  condenser,  giving  different  degrees  of 
obliquity. 

The  vertical  adjustment  is  obtained  by  sliding  in 
the  sub-stage. 

The  best  form  is  that  which  is  provided  with  an 
Iris  diaphragm,  which,  when  open,  gives  full  volume 
of  light  but  may  be  reduced  to  nearly  a  pin-hole 
opening.  Then  an  addition  which  carries  it  laterally 
under  the  condenser  by  rack  and  pinion,  and  another 
rack  and  pinion  for  vertical  adjustment  either  on  the 
sub-stage  bar  or  as  part  of  the  condenser.  As  the  suc- 
cessful utilization  of  the  condenser_depends  upon  the 
nicety  with  which  the  various  adjustments  can  be 
accomplished,  they  can  be  none  too  complete. 

Centering. — In  using  the  condenser  the  first  con- 
dition is  the  proper  centering.  For  this  purpose 
the  pin-hole  cap  should  be  placed  over  the  top  and 
focused  upon  by  means  of  a  medium 
power  objective.  The  condenser 
should  be  fixed  so  that  the  opening 
will  be  in  the  center  of  the  field.  For 
Fig.  24.  objectives  under  1.0  numerical  aper- 
ture it  is  not  necessary  to  bring  the  condenser  in 
immersion  contact  with  the  slide ;  for  objectives  of 
wider  aper-ture,  however,  it  should  be  done  by  first 
placing  a  drop  of  immersion  fluid  on  the  top  lens  and 
then  placing  the  slide  in  position  on  the  stage,  raise 
the  condenser  until  the  fluid  comes  in  contact  with 
the  lower  surface. 


105 

It  might  be  said  here  in  parenthesis  that  tl 
mirror  should  always  be  used  with  the  conde 

To  Focus  Condenser. — An  excellent  plan  for 
focusing  the  condenser  is  to  do  so  by  means  of  a 
low  power  objective,  f  to  3  inch,  in  the  microscope 
tube.  After  the  slide  is  in  contact  with  the  condenser 
or  when  not  used  with  immersion  and  close  to  it, 
focus  the  objective  on  the  object.  Now  adjust  the 
condenser  until  the  image  which  it  projects  of  the 
source  of  light  is  coincident  with  it.  In  the  case  of  a 
lamp,  the  flame  will  be  projected.  Daylight  offers  no 
tangible  image,  and  while  the  image  of  the  window- 
frame  is  not  really  quite  correct,  it  is,  in  most  cases, 
sufficiently  close  for  all  practical  purposes. 

Intensity  of  Light.— To  say  what  amount  of 
light  shall  be  used  is  very  difficult  on  account  of  the 
variety  of  specimens  which  involve  different  condi- 
tions. While  it  is  claimed  that  on  stained  bacteria 
the  best  plan  is  to  use  the  full  volume  of  illumination 
and  thus  differentiate  the  objects,  it  is  certainly  detri- 
mental to  do  so  on  many  histological  specimens,  diat- 
oms and  others,  as  they  would  be  drowned  in  light 
to  such  an  extent  as  to  be  in  some  cases  indistinguish- 
able. The  safest  plan  will  be  to  use  the  minimum  of 
light  and  gradually  increase  until  the  point  for  pro- 
per observation  is  reached.  It  will  also  be  a  good 
plan  not  to  use  more  light  than  is  needed  to  accom- 
plish the  purpose  desired. 

To  obtain  oblique  illumination,  reduce  the  aperture 
to  i  inch  and  gradually  bring  the  opening  out  of  the 


106 

center  up  to  the  limit  of  aperture  of  the  objective. 
When  beyond  this,  the  object  will  appear  illuminated 
on  a  dark  ground.  The  lateral  movement  should  be 
at  right  angles  to  the  striae  which  it  is  desired  to  see, 
and  if  this  is  not  known,  either  the  object  should  be 
revolved  or  the  mounting  of  the  condenser.  It  will 
sometimes  be  found  advantageous  to  reduce  or 
enlarge  the  opening. 


CARE  OF  A  MICROSCOPE. 


Besides  acquiring  the  ability  to  properly  use  an 
instrument  with  its  accessories,  it  is  important  to 
know  how  to  keep  it  in  the  best  working  condition. 
It  may  be  said  without  reserve  that  an  instrument 
properly  made  at  the  outset  and  judiciously  used 
should  hardly  show  any  signs  of  wear  either  in 
appearance  or  in  its  working  parts,  even  after  the 
most  protracted  use  ;  and  further  than  this,  every 
good  instrument  should  have  a  provision  for  taking 
up  lost  motion,  if  there  is  a  likelihood  that  this  may 
occur  in  any  of  the  parts. 

Especial  care  should  be  given  to  the  optical  parts, 
in  fact  such  care,  that  they  will  remain  in  as  good 
condition  as  when  first  received.  Accidental  injury 
may  of  course  occur  to  them,  but  if  a  systematic  man- 
ner of  working  is  followed  and  a  special  receptacle 
for  each  part  is  provided,  this  may  usually  be  avoided. 
The  following  rules  refer  mainly  to  the  instruments 
manufactured  by  the  Bausch  &  Lomb  Optical  Co., 
but  are  applicable  to  instruments  in  general. 


108 

To  Take  Care  of  a  Stand.— One  of  the  first 
rules  should  be  to  keep  the  instrument  free  f ram  dust. 
This  may  be  done  in  a  manner  formerly  prescribed. 
If  dust  settles  on  any  part  of  the  instrument,  remove 
it  first  with  a  camel's  hair  brush,  and  then  wipe  care- 
fully with  a  chamois  skin,  with  the  grain  of  the  finish 
and  not  across  it,  as  in  the  latter  case  it  is  likely  to 
cause  scratches.  Keep  the  working  and  sliding  parts 
absolutely  free  from  dust,  as  this  grinds  and  will 
thus  soon  cause  play. 

Use  no  alcohol  on  any  part  of  the  instrument,  as  it 
will  remove  the  lacquer.  As  the  latter  is  for  the  pur- 
pose of  preventing  oxydation  of  the  metals,  it  is 
important  to  observe  this  rule. 

In  using  the  draw-tube  impart  a  spiral  motion.  In 
instruments  which  have  no  cloth  lining,  a  straight  up 
and  down  movement  should  be  employed,  as  the  tube 
will  otherwise  become  scratched. 

If  it  becomes  necessary  to  lubricate  any  of  the 
parts,  use  a  slight  quantity  of  soft  tallow  or  good 
clock  oil. 

In  an  instrument  which  is  in  constant  use,  it  some- 
time occurs  that  the  pinion  works  loose  and  occasion- 
ally to  such  an  extent  that  the  body  drops  of  its  own 
weight.  Tightening  screws  are  provided  to  take  up 
the  play — in  the  Professional,  American  Concentric, 
Universal,  Physician,  Biological  and  Library  Micro- 
scopes these  are  in  the  back  of  the  pinion.  In  the 
Investigator,  Model  and  Family  Microscopes,  they 
are  seen  in  the  slide  by  removing  the  body. 


109 

In  using  a  screw-driver,  grind  its  two  large  sur- 
faces so  that  they  are  parallel  and  not  wedge-shape, 
and  so  it  will  exactly  fit  in  the  slot  of  the  screw-head. 

In  inclining  the  stand  always  grasp  it  at  the  arm 
and  never  at  the  tube,  as  in  the  latter  case  it  may 
loosen  the  slide  or  tear  off  some  of  the  parts. 

When  repairs  or  alterations  are  necessary,  always 
have  these  made  by  the  manufacturers  ;  they  can, 
from  the  system  of  duplicated  parts,  not  only  do  it 
cheapest,  but  best. 

To  Take  Care  of  Objectives  and  Eye- 
Pieces. — It  is  as  necessary  to  keep  these  free  from 
dust  as  the  stand,  in  fact  even  greater  cleanliness 
should  be  observed.  When  indistinct,  dark  specks 
show  in  the  field,  the  cause  may  usually  be  looked  for 
in  the  field-lens,  although  sometimes  in  the  eye-lens 
also.  The  dust  may  be  removed  by  a  camel's-hair 
brush,  but  when  this  is  not  sufficient  use  a  well  washed 
piece  of  linen,  such  as  an  old  handkerchief.  From 
its  fine  texture  chamois  skin  is  desirable,  but  as  it  is 
fatty  it  should  never  be  used  until  after  it  has  been 
well  washed. 

The  same  method  applies  to  cleaning  objectives. 
Clean  an  immersion  objective  invariably  after  it  has 
been  used,  first  by  removing  the  fluid  by  a  moist  linen 
and  then  by  using  a  dry  piece. 

Keep  the  objectives  especially  in  such  a  place 
where  they  are  not  subject  to  extreme  and  sudden 
changes  of  temperature,  as  the  unequal  expansion 


110 

and  contraction  of  glass  and  metal  may  cause  the 
cement  between  the  lenses  to  crack.  Also  keep  them 
from  direct  sunlight. 

Screw  them  into  the  nose-piece  and  unscrew,  by 
grasping  the  milled  edge. 

Avoid  any  violent  contact  of  the  front  lens  with 
the  cover-glass.  Usually  the  latter  suffers,  but  it  is  as 
liable  to  occur  to  the  former. 

Above  all,  it  should  be  made  a  rule  that  no  one  but 
the  owner  handle  the  microscope  and  accessories. 
One  person  may  be  expert  in  the  manipulation  of  one 
instrument  and  still  find  it  difficult  to  work  with 
another.  The  fine  adjustment  particularly  causes  the 
greatest  difficulty  as  in  some  instruments  it  corres- 
ponds with  the  movement  of  the  micrometer  screw, 
while  in  others  it  is  contrary  and  thus  the  objective 
as  well  as  object  are  endangered. 


APPENDIX. 


(Reprinted  from  "The  Microscope,"  Jan.,  1885.) 


CONSIDERATIONS  IN  TESTING  OBJECTIVES. 


EDWARD    BAUSCH. 


There  is  a  laudable  desire  in  almost  all  persons 
possessing  a  microscope  to  become  intimately  ac- 
quainted with  it,  and  for  this  purpose  it  is  not  only 
necessary  to  learn  the  use  of  its  mechanical  parts, 
but  to  understand  its  optical  capacity,  which  is  .con- 
siderably more  difficult,  and  which  involves  more 
considerations  than  would  appear  on  first  thought. 

With  all  the  care  which  may  be  bestowed  upon 
objectives,  they  are,  to  a  certain  extent,  works  of 
chance,  and  depend  upon  the  optician's  judgment, 
industry  and  skill,  and  upon  the  variations  in  glass, 
for  their  excellence  and  uniformity.  These  condi- 
tions are  often  so  varying  that  in  the  case  of  sev- 


eral  objectives  of  the  same  formula,  made  at  the 
same  time  there  will  be  such  great  differences  that 
it  can  hardly  be  conceived  on  the  first  examination, 
that  they  were  to  be  similar.  It  is  at  this  point 
especially  necessary  to  detect  the  errors,  to  deter- 
mine their  cause  and  apply  the  remedy,  and  to  do  this 
properly  often  involves  an  inconceivable  amount  of 
work,  and  in  many  cases  the  final  results  are  reached 
at  a  pecuniary  loss.  There  are  certain  fixed  tests  for 
each  kind  of  objective,  and  to  the  best  of  my  knowl- 
edge all  reputable  opticians  bring  each  objective  up 
to  its  standard  before  allowing  it  to  pass  their  hands, 
irrespective  of  the  cost  of  doing_so.  This  must  of 
necessity  be  so,  if  only  out  of  business  consideration, 
and  not  for  a  love  of  each  production,  for  it  is  evi- 
dent that  a  well-earned  reputation  would  soon  lose  its 
pre-eminence,  and  would  acquire  one  for  unreliable  or 
poor  work,  if  on  comparison,  objectives  of  the  same 
kind  would  show  a  marked  difference.  There  is  some- 
times a  fortunate  combination  of  circumstances  which 
makes  a  certain  objective  better  than  its  fellows,  but 
this  is  a  rare  exception,  and  is  positive  evidence  that 
the  exact  requirements  of  the  formula  have  been  com- 
plied with.  As  a  rule,  therefore,  I  believe  that  the 
opticians'  claim  may  be  relied  upon,  and  where  the 
results  in  the  hands  of  the  microscopist  do  not  corres- 
pond with  them,  .the  cause  may  usually  be  looked  for 
in  the  lack  of  experience  in  manipulation  or  in  con- 
ditions, which  differ  from  those  under  which  the  objec- 
tive was  completed.  The  belief,  which  I  am  aware  is 


113 

extant,  that  there  are  great  differences  in  objectives 
purporting  to  be  similar,  is,  in  my  opinion,  not  justi- 
fied, at  any  rate  in  the  productions  of  those  men  who, 
by  general  acknowledgement,  are  at  the  head  of  their 
profession.  I  admit  that,  as  in  everything  which 
depends  upon  human  skill,  there  is,  strictly  speaking, 
no  absolute  uniformity,  but  also  claim,  that  with  few 
exceptions,  the  differences  are  so  slight,  that  anything 
but  the  most  expert  manipulation  cannot  detect  them. 

It  therefore  appears  to  the  writer  that  any  informa- 
tion which  will  tend  to  improve  the  knowledge  of 
testing  objectives  will  not  only  prove  beneficial  to  the 
microscopist,  but  will  prove  advantageous  to  the 
optician,  in  that  his  work  will  receive  a  fair  trial,  based 
upon  a  knowledge  of  the  principles  involved,  and 
that  he  may  be  convinced  that  all  his  work  which 
deserves  commendation  will  be  the  better  appreciated. 
The  following  points  are  by  no  means  new,  but  are 
often  lost  sight  of  in  making  tests.  The  writer  will 
speak  of  medium  and  high  power  objectives  only,  as 
the  deleterious  influences  are  most  noticable  in  these, 
but  they  apply  as  well  to  the  lower  powers  though  in 
a  less  degree. 

The  part  of  the  instrument  which  has  a  strong 
bearing  in  the  performance  of  the  objective  is  the 
mirror.  It  should  be  adjustable  on  the  mirror-bar,  so 
that  it  can  be  accommodated  to  the  variations  in  dis- 
tance of  the  source  of  light  from  the  instrument. 
When  parallel  rays  are  used,  as  with  light  from  the 
sun  or  clouds,  its  distance  from  the  object  should  be 


114 

decreased  and  increased  when  lamp-light  is  used.  It 
should  be  exacted  that  the  focus  of  the  concave  mirror 
be  within  the  limits  of  its  adjustment.  The  serious 
disadvantage  of  its  incorrectness  in  this  respect  can 
easily  be  seen  by  taking,  for  instance,  a  -^  objective 
which  will  resolve  P.  angulatum  nicely  with  central 
light,  when  the  mirror  is  exactly  focused.  By  mov- 
ing the  latter  out  of  focus  it  will  be  seen  that  the 
objective  loses  in  performance,  and  if  this  is  carried 
sufficiently  far  it  will  arrive  at  a  point  where  the 
objective  will  cease  to  show  any  lines.  The  effect  will 
be  the  same  on  any  other  object,  and  is  caused  by  the 
lack  of  proper  concentration  of  light  on  the  slide. 
When  oblique  light  is  used,  unless  the  diaphragm 
moves  with  the  mirror,  it  should  be  removed,  as  the 
advantage  of  obliquity  is  diminished  or  destroyed  by 
the  loss  of  light. 

The  cover-glass  exerts  probably  the  greatest  influ- 
ence in  testing  as  well  as  in  general  work.  This 
should  be  used  of  a  thickness  which  corresponds  to 
that  to  which  the  objective  (if  non-adjustable)  was 
originally  corrected.  If  thicker  or  thinner  covers  be 
used,  the  objective  will  be  spherically  over  or  under 
corrected,  and  will  have  to  be  moved  correspondingly 
above  or  below  the  plane  ( outline )  of  the  object 
to  distinguish  its  structure,  if  the  variation  is  con- 
siderable the  difference  between  the  two  planes  will 
be  so  great  that  it  will  cease  to  show  any  structure, 
and  it  may  then  be  said  to  be  lacking  in  defining 
power,  although  in  reality  it  possesses  it  but  is  not 


115       , 

properly  used.  Generally  speaking  the  objective  may 
be  said  to  be  spherically  corrected  when  it  gives  the 
best  defined  image  ;  that  is,  when  the  outline  and 
internal  structure  of  an  object  of  extreme  thinness 
appear  in  one  plane.  When,  after  the  objective  has 
been  focused  on  the  outline  of  the  object,  it  is  neces- 
sary to  increase  the  distance  to  focus  on  the  structure, 
it  is  evidence  that  the  objective  is  spherically  over- 
corrected  and  that  the  cover  is  too  thick  ;  in  adjust- 
able objectives  the  correction  collar  must  be  brought 
to  its  closing  point,  which  means  that  the  lenses  are 
brought  in  closer  contact.  When  the  objective  must 
be  focused  to  a  point  beyond  the  outline  of  the  object 
to  see  its  structure— that  is,  brought  closer  to  the 
cover-glass — it  proves  that  this  is  too  thin,  and  is  then 
said  to  be  spherically  under  corrected  ;  to  give  the 
proper  adjustment  in  an  adjustable  objective  in  this 
case  the  adjustment  is  opened — the  lenses  are  sepa- 
rated. It  requires  a  certain  amount  of  study  to  dis- 
tinguish these  phenomena,  and  although  it  can  be 
done  in  well  prepared  specimens,  I  know  of  none 
better  than  coarsely  marked  diatoms,  such  as  P. 
angulatum. 

Although  I  am  aware  that  many  eminent  micro- 
scopists  do  not  favor  adjustable  objectives  for  every 
day  work,  1  must  confess  that  I  fail  to  see  the  force 
of  their  arguments.  From  the  foregoing  it  will  be 
seen  that  unless  the  cover-glasses  are  of  a  thickness 
corresponding  to  that  which  was  originally  used,  the 
objective  may  be  made  to  do  imperfectly  what  is  in  its 


116 

power  to  do  well,  and  when  pressed  to  its  full  capacity 
may  and  is  likely  to  fail.  It  must  be  remembered 
that  cover-glasses  of  the  same  number  are  not  of  the 
same  thickness.  The  selection  of  those  of  proper 
thickness  is  expensive  and  tedious,  whereas  the 
knowledge  of  correcting  the  objective  is  easily 
acquired,  and  in  the  latter  case  it  is  in  the  manipu- 
lator's power  to  command  the  highest  performance  of 
which  the  objective  is  capable  ;  further  than  this,  it 
has  the  advantage  that  it  may  be  used  as  a  non-adjust- 
able objective  if  desired.  When  homogeneous  immer- 
sion objectives  were  first  introduced  they  were  mounted 
in  fixed  settings,  as  it  was  expected  that  the  thickness 
of  the  cover-glass  would  not  affect  the  correction  ; 
although  this  assumption  was  correct,  it  was  found 
that  even  in  these  it  was  necessary.  How  much  more 
then,  is  it  required  in  dry  or  water  immersion  objec- 
tives ? 

Another  factor  in  the  disturbing  influences  is  the 
variation  in  length  of  tube  ;  the  deleterious  results 
are  similar  to  those  with  varying  cover-glasses.  Objec- 
tives are  usually  adjusted  to  8|  or  9  inches  length  of 
tube,  and  although  this  in  itself  is  a  fixed  standard,  it 
usually  becomes  variable  by  changing  objectives  and 
eye-pieces.  That  this  is  so  in  objectives  is  patent, 
and  that  it  is  so  in  eye-pieces  can  easily  be  determined 
by  making  a  change  in  powers,  when  it  will  be  found 
that  a  change  in  focus  is  required.  By  decreasing  the 
length  of  tube  the  objective  will  appear  to  be  spheri- 
cally under-corrected  and  vice  versa  when  it  is  increased, 


117 

so  that  it  is  apparent  that  by  the  use  of  the  draw-tube 
the  effect  of  the  cover-glass  may  be  partially  neutral- 
ized ;  for  instance,  when  by  the  use  of  a  thin  cover 
the  objective  is  spherically  under-corrected,  it  may,  to 
a  certain  extent,  be  corrected  by  causing  a  correspon- 
ding over-correction  in  the  tube  by  increasing  its 
length.  The  use  of  the  draw-tube  for  the  purpose  of 
changing  the  amplification  or  for  the  matter  of  con- 
venience can  hardly  be  commended,  except  in  cases 
where  adjustable  objectives  are  used. 

Considerable  also  depends  upon  the  perfection  of 
the  eye-piece.  I  believe  that,  as  a  rule,  too  little  care 
is  devoted  to  it  ;  at  any  rate,  it  is  certain  that  while 
any  Huyghenian  eye-piece  for  a  telescope  can  be  used 
on  a  microscope,  very  few  which  have  been  made  for 
this  can  be  used  on  a  telescope  ;  and  while  it  is  true 
that  no  such  perfection  may  be  required  in  the  former, 
it  leaves  such  an  indefinite  range  that  it  may  become 
difficult  to  place  a  limit  for  the  perfect  and  imperfect. 
In  all  work,  and  especially  in  testing,  it  should  be  seen 
that  the  eye-lens,  as  well  as  the  field-lens,  are  perfectly 
clean. 

Among  the  absolutely  necessary  conditions  in  judg- 
ing of  the  quality  of  an  objective  are  perfect  speci- 
mens, especially  if  they  are  sections.  A  thick  object 
obstructs  the  light  and  generally  makes  it  necessary 
to  go  through  so  many  layers  or  planes  that  it  is  diffi- 
cult to  get  any  one  distinct ;  the  impression  may  thus 
easily  be  given  that  the  objective  is  at  fault.  The 
difference  between  two  objects  of  the  same  nature 


118 

may  be  so  great  that,  while  with  one  the  objective  may 
be  condemned  as  imperfect,  it  may  with  the  other 
appear  t»o  be  of  extraordinary  excellence. 

In  conclusion,  I  will  say  that  there  may  be  other 
conditions  which  may  influence  the  performance  of  a 
lens,  and  to  acquire  the  power  of  eliminating  them 
requires  considerable  experience.  When  an  objective 
does  not  correspond  with  the  claims  of  the  optician, 
judgment  should  not  be  passed  upon  it  until  after 
repeated  trials  have  been  made,  in  all  of  which  the 
above  points  should  not  be  lost  sight  of. 


Bausch  &  Lomb  Optical  Co., 

MANUFACTURERS    OF 

MICROSCOPES,  OBJECTIVES,    ACCESSORIES, 

PHOTOGRAPHIC    LENSES,   DIAPHRAGM 

SHUTTERS,  TELESCOPES,  EYEGLASSES  AND 

LENSES,  MAGNIFYING  GLASSES, 

AND    A   LARGE   VARIETY    OF 

OTHER   OPTICAL    INSTRUMENTS. 


FACTORY  AND  MAIN  OFFICE, 

531  TO  543  NORTH  ST.  PAUL  STREET, 

ROCHESTER,  N.  Y. 


BRANCH  OFFICE, 

48  AND  50  MAIDEN  LANE, 

NEW  YORK  CITY. 


120 


On  the  following  pages  will  be  found  illustrations 
of  a  number  of  instruments  manufactured  by 

Bauseh  &  Lomb  Optical  Co., 

ROCHESTER,  N.  Y.,  AND  NEW  YORK  CITY, 

Who  issue  the  following  Catalogues  : 

I — Eyeglasses,  Lenses,   Magnifiers,   Readers,  etc. 

2 — Microscopes,  Objectives  and  Accessories. 

3 — Supplement    to     Microscope    Catalogue    (i2th 
Edition). 

4 — Photographic  Lenses  and  Diaphragm  Shutters. 

Any  or  all  of  which  will  be  sent  to  any  address 
free  on  application. 


121 


AMERICAN  STANDARD 

PHOTOGRAPHIC  LENSES, 


UNIVERSAL 
G.  CLARK. 


Both  conceded  to  be,  at  least,  equal   to  the   best 
of  European  manufacture. 


122 


(Cut  one-third  of  actual  size.) 

COMPACT  DISSECTING  AND  MOUNT 
ING   MICROSCOPE. 


123 


(Cut  one-third  of  actual  size.) 

MODEL   MICROSCOPE, 


124 


Made  in  three  forms. 

ist  with  sliding  tube,  up- 
right. 

2d  with  sliding  tube  and 
joint  for  inclination. 

3d  with  rack  and  pinion  for 
coarse  adjustment  and  joint 
for  inclination. 


' r:  =,  i 


(Cut  one-third  of  actual  size.) 

BIOLOGICAL   MICROSCOPE. 


125 


(Cut  one-third  of  actual  si/e.) 

PHYSICIAN    MICROSCOPE 


126 


(Cut  one-third  of  actual  size.) 

UNIVERSAL   MICROSCOPE. 


127 


BRANCH   OKFICB 

OF    THE 

Bauseh  &  Lomb  Qptieal  Co., 

IS    LOCATED    AT 

48  AND  50  MAIDEN    LANE, 
NEW  YORK   CITY, 

Where  all  visitors  are  cordially  invited  to  call,  and 
will  find  in  stock  a  complete  assortment  of  goods 
of  our  manufacture  which  will  be  shown  with 
much  pleasure. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

IS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 

Books  not  returned,  on  time  are  subject  to  a  fine  of 
50c  per  volume  after  the  third  day  overdue,  increasing 
to  $1.00  per  volume  after  the  sixth  day.  Books  not  in 
demand  may  be  renewed  if  application  is  made  before 
expiration  of  loan  period. 


OCT  28 1918 


SE? 


50m-7,'16 


Sccfl 


